Tyrannosaurus, Osborn, 1905

Osborn, Henry Fairfield, 1912, Crania of Tyrannosaurus and Allosaurus, Memoirs of the American museum of Natural History 1, pp. 1-30 : 6-44

Skull of Tyrannosaurus .

Unless otherwise stated the following description refers to the most perfect skull No. 5027 Amer. Mus. Collection .

In general the skull is by far the most powerful and massive known among the reptiles. Among the living Reptilia it has remote resemblances with the skulls of Sphenodon (temporal region) and of Crocodilus (occipital and basisphenoidal region); among the extinct Reptilia its closest resemblances are with the skulls of other Theropoda and of the Sauropoda, especially in four points, namely:

(1) Abbreviation of the cranial region.

(2) Elongation of the facial region.

(3) Multiple fenestration of the facial region.

(4) Upward flexure of the brain case.

The palate displays one feature which is very important, namely, the union of the vomers into a single diamond-shaped plate articulating by a long, narrow process with the pterygoids posteriorly. This is a strong avian analogy as seen by comparison with the primitive ratite palate of Dromoeus ( Fig. 5 ). This resemblance was first pointed out to the author by his friend Mr. Pycraft of the British Museum.

Another striking feature is the deep downward prolongation of the basioccipital-basiphenoidal plate, in which these tyrannosaurs contrast widely with all recent reptiles, including the Crocodilia, but agree with the Sauropoda.

Chief Measurements of Skull ( Amer. Mus. No. 5027 ).

1. Basilar length, premaxillaries to occipital condyles	1.210m.	48 in.
2. Total width across quadrato-jugals
(a) As estimated from transverse measurements in the crushed condition	835	33
(b) As estimated by doubling the width of the nearly normal right side	904	35f
3. Total height, top of occipital crest to bottom of the quadrate bone	.635	25
4. Length along side of skull, premaxillaries to back of paroccipital processes	1.355	CO
5. Length of lower jaw, symphysis to back of articulars	1.205	48

Apparently the above measurements of No. 5027 are almost identical with those of the type skull ( No. 973 ). The cranium No. 5117 belongs to a smaller and younger animal in which the sutures are more distinct.

It will simplify the description to consider the different aspects of the skull in consecutive order.

Lateral View, Skull No. 5027 .

Text Fig. 1 and Pl. I .

The skull and jaw combined present deep and massive rather than elongate proportions. The areas of muscular attachment are heavily rugose. The rugose area extends also down the superior surface of the nasals.

Parietal, summit of crest only observed in lateral view.

Paroccipital (process) or opisthotic (op. o.), extreme lateral portion projecting behind the squamosal.

Squamosal, with forwardly projecting bars and uniting respectively with the postorbital and the superior surface of the quadrato-jugal.

Quadrato-jugal, with broad superior plate for inferior bar of squamosal, overlapping outer portion of quadrate and with an inferior bar overlapping the jugal.

Jugal of triradiate shape with anterior bar uniting with maxillaries and adlachrymals, perforated by a small fenestra (jf.); the superior bar uniting with the postorbitals, the posterior bar uniting with the quadrato-jugals.

Pterygoid, superior surface of palatal portion seen through the first antorbital fenestra ( Pl. II ).

Postorbital, probably a complex of postfrontals and postorbitals, with an inferior jugal and a posterior, squamosal process. The orbital rugosity (orb. rug.) rises immediately behind a deep notch which separates this element from the adlachrymal. The orbit is partly bounded below by an anterior process of the postorbital.

Prefrontals and frontals visible only on top of skull.

Adlachrymal ('‘ La'.) (= “ lachrymals” of authors), greatly enlarged, with broad anterior process uniting with nasals and maxillaries, inferior process uniting with jiigals. The supraorbital rugosity of the adlachrymal is separated by the deep notch from the rugosity of the supraorbital; the upward and backward growth of the adlachrymal has shut the frontal, pre- and postfrontal out of lateral view. No sutures remain between the adlachrymal and postorbital.

Nasal, uniting with adlachrymals, maxillaries, and premaxillaries.

Maxillary fenestrated, suturally uniting with premaxillaries, with nasals, but not with frontals.

Orbit, reduced in size, surrounded by adlachrymals, postorbitals, and partly enclosed below by anterior bar of postorbitals.

Nares, elongate, oval.

Fenestration.

Beside the orbital and narial openings, we observe five openings, on the side of the skull, namely:

(1) Latero-temporal fenestra, partially divided by the horizontal squamosal-quadratojugal bar.

(2) Infraorbital fenestra restricted off from the orbital fenestra proper.

(3) Small, round jugal fenestra (j.f.). (4) Large antorbital fenestra (ant.f.). (5) A smaller second antorbital fenestra (ant.f."). (6) Diminutive maxillo-premaxillary fenestra (ant.f."'). Superior View, Skull No. 5117 .

Text Fig. 2 and Pl. II.

The superior view of the skull brings out with great clearness the relations of pairs of the same bony elements in the facial and cranial regions, especially the peculiar relations of the adlachrymals, postorbitals, and greatly reduced and constricted prefrontals, which are thrust in toward the median line of the skull instead of occupying the free lateral supraorbital position which is observed in Allosaurus .

Parietals, rising into an antero-posterior sagittal crest and a transverse supraoccipital crest.

Frontals, abbreviated, suturally separate from each other and from surrounding elements in young individuals; in adults closely united laterally with postorbitals. Anteriorly uniting with nasals, prefrontals, and adlachrymals. Suture between frontals and nasals not apparent in adult skull.

Postorbitals, with prominent rugosity (orb. rug.), immediately in front of which is the deep, narrow groove separating this element from the adlachrymal rugosity. This deep groove or fossa is apparently a vestige of the opening or cleft above the orbit in Allosaurus ( Fig. 26 ), in which the prefrontals are displayed.

Prefrontals (Pr.fr.), with very limited, wedge-shaped exposure on top of the skull.

Adlachrymals (‘ La’.) greatly expanded.

Nasals narrow, median area coalesced into a single rugose plate, suturally separate anteriorly and posteriorly, uniting by very narrow sutural contact with frontals posteriorly, and with premaxillaries anteriorly.

No pineal opening is observed at the top of the skull; the sagittal crest, formed of the frontals and parietals, is solid.

Fenestration.

The following openings are observed:

(1) The large supratemporal fenestra (sup.fen.). (2) The lateral temporal fenestra (lat.fen.). (3) The orbito-infraorbital opening.

(4) The first antorbital fenestra (ant.f.). (5) The second antorbital fenestra (ant.f''.). (6) The third antorbital fenestra (ant.f'''.).

(7) The anterior nares (ant.nar.).

Anterior View, Skull N o. 5027 .

Text Fig. 3 .

The anterior views of skull shown in Fig. 3 serve to bring out still more clearly the relations of the same cranial elements. From behind forward the median line is seen to be composed in pairs of the parietals, frontals, nasals, and premaxillaries.

Prefrontals (Pr. fr.), wedged in between the frontals and adlachrymals.

Adlachrymals (‘ L a’), form vertical buttresses between the orbital and antorbital openings.

Premaxillaries, suturally separate, each containing four teeth.

Dentaries, freely apposed, i. e., not united in median line.

Occipital View, Skulls Nos. 5027 , 5117 .

Text Fig. 4 .

The relations of pairs of bones of the occipital region are perfectly displayed in No. 5027 ( Fig. 4 ) and in the younger specimen No. 5117 .

Parietals, vertically and horizontally expanded, forming the entire upper portion of occiput,

impressed posteriorly by median pits for the attachment of the ligamentum nuchce, extending laterally over the paroccipital processes (Op.o.); between this parietal spur and the.paroccipital process may be observed the vestigial posttemporal fenestra (p. temp. fen.).

Supraoccipitals, relatively reduced in size, coalesced inferiorly with exoccipitals, which probably bridge over the foramen magnum as in Allosaurus , coalesced laterally with paroccipital or opisthotic elements.

Basíoccípítal, composing the broadly oval condyle (con), sending down a flange inferiorly into the basioccipital plate (B. oc.), spreading laterally to coalesce with the paroccipital processes (Op. o.) or opisthotics.

Paroccipitals, or opisthotics (Op. o.) coalescing toward median line with exoccipitals and basioccipitals, expanding laterally and articulating with squamosals.

Basisphenoids, projecting downward below level of basioccipitals (compare Pl III and Text Figs. 7 and 16 ); presenting inferior lateral processes for articulation with the pterygoids; homologous with the “ basipterygoid ” processes of Sphenodon and lizards.

Pterygoids, uniting by movable joints with “ basipterygoid” process of basisphenoid; uniting laterally by a broad suture with the quadrate. (This suture is shown on the left hand side of the occiput in Fig. 4 but not on the right side.)

Quadrates, broad posteriorly, firmly united by suture with pterygoids anteriorly; suturely united with quadrato-jugals exteriorly except at the point of the large quadrate foramen (f. qu.).

The most important feature of Tyrannosaurus as distinct from Allosaurus is that the quadrate is closely united with the squamosals above and by suture instead of presenting a ginglymoid, or hinge relation as in Allosaurus .

Fenestration.

Three pairs of fenestrse appear at the back of the skull, namely:

(1) vestigial posttemporal fenestrse,

(2) very constricted openings between the parietal crest and the squamosals leading into the supratemporal fenestrse.

(3) the fenestrse (f. qu.) between quadrate and quadrato-jugal.

Palatal View, Skull No. 5027 .

Text Figs. 5 , 6 .

All the elements of the palate are clearly displayed in the natural position excepting the ectopterygoids, which are thrust backward from their natural junction with the maxillaries, and the parasphenoidal elements in front of the basisphenoid (Pa. sp.), which are buried in the matrix. The relations of the bones of this parasphenoidal portion of the skull are seen in the young specimen No. 5117 , as displayed in Figs. 7 , 8 , in which the relations of the parasphenoid to the basisphenoid are shown; the space marked Pa. sp. in Fig. 6 is partly occupied by the parasphenoid, a long, splint-like element.

Of especial interest is the analogy which the palatal view of Tyrannosaurus exhibits to that of the Cassowary ( Fig. 5B ).

The striking features of the palate are:

(1) The single, diamond-shaped, birdlike vomer, which is to be contrasted with the elongate, diamond shaped vomer of Dromceus.

(2) The posterior nares bounded by the vomers, maxillaries, and palatines.

(3) The rows of eleven interdental expansions of the dental alveoli of the maxillaries. Each of these expansions may be known as a rugosa; its function is apparently to bear a portion of the lateral strain on the great teeth. It is noteworthy that these interdental ‘ rugosae ’ are exposed in the palate, whereas in the lower jaw they are covered by a prolongation of the splenials, known as the supradentary plate.

The palate is composed of the following elements:

Premaxillaries, each containing four teeth, between which lie four more or less sharply defined interdental rugosse.

Maxillaries, unite in a typical manner with the premaxillaries, vomers, palatines, ectopterygoids, and jugals.

Vomers, with anterior diamond-shaped expansion, without median suture, prolonged posteriorly to divide the posterior nares (p. nar.) and unite with slender anterior spurs of the pterygoids.

Palatines, bounding the posterior nares laterally, uniting with maxillaries, pterygoids, vomers, and jugals.

Pterygoids, very large and complex, laterally uniting with the ectopterygoids (displaced in No. 5027 ), anteriorly with the vomers and palatines, posteriorly with the basisphenoids and with the quadrates by broad pterygo-quadrate sutures.

Ectopterygoids, suturally uniting with pterygoids and loosely united externally with maxillaries and possibly in contact with the jugals. The ectopterygoid of the right side is seen displaced within the upper part of the left antorbital fenestra ( Plate I ).

Occiput and Brain Case with Temporal Arches Removed, No. 5117 .

Text Figs. 7 , 8 ; and Pll. III , IV .

The outer structure of the brain case proper is best shown in the cranium of the relatively young tyrannosaur ( No. 5117 ), collected and presented to the Museum by Charles H. Sternberg. The absence of the temporal arches freely displays the component elements of the cranium and the exits for the various cranial nerves. The homologizing of these elements and identification

of the nerve exits has been a very difficult undertaking, for which Mr. Bamum Brown, Dr. W. K. Gregory, and Dr. F. von Huene are chiefly to be credited.

Parietals, with broadly expanding posterior plate, narrow and depressed median crest.

Frontals, abbreviate, suturally separate from parietals and from prefrontals.

Prefrontals (Pr. f.) greatly reduced, wedged in between frontals and nasals (Na.), only the posterior part of which are shown.

Ethmoid complex uniting suturally above and laterally with prefrontals and frontals, posteriorly with (?) presphenoids (P. sp.); in anterior view ethmoid complex ( Fig. 7 ) with a median septum dividing olfactory lobes ( Fig. 8 ).

? Presphenoids (P. sp.) imperfectly separated from parasphenoids, or parasphenoid region, below; bounded anteriorly by ethmoids, superiorly by frontals, posteriorly by orbitosphenoids (see obscure sutures in Fig. 7 ).

Parasphenoids (Pa. sp.), supporting ethmoid above, coalescing with?presphenoids posteriorly and descending as a vertical plate ( Figs. 7 , 8 ) to bifurcate on the anterior surface of orbitosphenoids and basisphenoids.

Orbitosphenoids (0. sp.) (= alisphenoids of authors), very large elements, uniting above with?presphenoids, frontals, parietals, posteriorly with prootics, exoccipitals, inferiorly overlapping basisphenoids; perforated by the following cranial nerves, ii, iii, iv, vx, v2, v3; anterior border of the orbitosphenoids (compare Pl. III , IV ) invaginated by the pituitary fossa (pit. fos.).

The relations of the orbitosphenoids of Tyrannosaurus ( Figs. 7 , 8 and Pl I. Ill , IV ) are similar to those of Crocodilus ( Figs. 14 , 15 ) which have been worked out for comparison and drawn under the direction of Dr. W. K. Gregory.

Basisphenoids (B. sp.), extending down below the level of the basioccipital ( Figs. 7 , 8 ), unlike the condition in the Crocodilia ( Figs. 14 , 15 ).

The antero-inferior border is indented by the internal carotid foramen (Car. in.).

Prootics, widely exposed, bounding auditory fenestra (fen. ov.), superiorly, uniting with orbitosphenoids anteriorly, with parietals superiorly, with opisthotics laterally, with exoccipitals posteriorly ( Figs. 7 , 8 and Pl I. Ill , IV ).

Paroccipitals, or opisthotics (Op. o.), perforated by external auditory opening, or fenestra ovalis. Near the fenestra ovalis is a minute opening for the transmission of the seventh nerve (vii).

Mid - cranial Section, Skull No. 5029 .

Pll. III , IV .

Taken with the previous description of the external elements of the cranium, the mid-cranial section of another skull, Amer. Mus. No. 5029 , as represented in the beautiful drawings of Mr. Christman in Pll. III , IV , completes the anatomy of the brain case, and presents the brain cavity and exits of the cranial nerves from within.

This specimen displaying the very massive roof of the brain case composed of the supraoccipitals, parietals and frontals, and massive floor of the medulla oblongata composed of the orbitosphenoids, basisphenoids and basioccipitals, may be compared with the similar parts in the skull of Crocodilus ( Figs. 14 , 15 ), from which it appears that the anterior portion of the brain-case is relatively weak and possibly in part membranous, e. g., anterior wall of pituitary fossa (pit. fos.).

The latter contrasts with the solid boundaries of the pituitary fossa in the cranium of Sauro­ poda ( Fig. 16 , Diplodocus ). The large (?)pineal fenestra pin.) in the roof of the Diplodocus cranium has no counterpart in the massive parieto-frontal union of Tyrannosaurus . The two chief contrasts with the Sauropoda cranium are, therefore, (1) absence of pineal fenestra, (2) absence of heavy bony wall of infundibulum.

The inner aspect of the cranial cavity of Tyrannosaurus displays the following elements:

? Presphenoids not suturally distinct posteriorly from orbito-sphenoids. Perforated by nerve II.

Orbitosphenoids perforated by openings of III, IV, V1, V2, Vs; inferiorly there is a bridge of bone separating the exit of the optic nerves (II) from the opening of the infundibulum (p.f.); orbito-sphenoids uniting posteriorly with prootics.

Prodtics (Pr. o.), coalesced with surrounding elements, transmitting cranial VIII.

Opisthotic (Op. o.) + exoccipital (Ex. oc.) complex, transmits cranial nerves IX-XI, XII.

Basisphenoid containing a large cavity or recessus at first mistakenly re­ garded as lodging the pituitary body. No opening is found for the abducens nerve (VI), which probably pierced the floor of the basisphenoid near the midline, as in Crocodilus .

Brain Cavity, Skull No. 5029 .

Pll. III , IV .

Comparison of the intracranial cavity of Tyrannosaurus With the mid-section of the skull of Sphenodon and brain in situ as figured by Dendy 1 (1910, pl. xix, fig. 1), shows that the intra- cranial cavity in Tyrannosaurus corresponds with the outer surface and foldings of the dura mater and is thus merely a cast of the outer envelope of the brain, which gives us little idea either of the form or the size of the brain itself. The reasonable inference is that the intracranial cast of Tyrannosaurus greatly exceeds and possibly doubles in cubic capacity the actual brain which was formerly contained within it.

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In Sphenodon the cubic capacity of the dura mater envelope ippears to be double that of the brain itself. Thus the cast of Tyrannosaurus ( Fig. 17 ; Pl. III ) gives us a means of measuring the size of the dura mater envelope. It displaces 530 cubic centimeters Jf water. If the brain proper bore the same proportion to the iura mater envelope as that of Sphenodon , the bulk of the brain of Tyrannosaurus may be estimated at 250 cubic centimeters. The brain proper was extremely small in comparison with the enormous size of the body.

Dendy observes (op. cit., pp. 236): “Hence it comes about that the brain occupies not nearly the whole of the cranial cavity, being suspended in it by innumerable thin strands of connective tissue which extends radially across the sub-dural space and con- nect the dura mater with the pia, Which latter closely invests the brain..I may be allowed to lay stress on the extraordinary dis- parity between the size of the brain and that of the cranial cavity. It follows that the shape of the latter can afford no reliable indi- cation of that of the former. The same is probably true of many fossil reptiles, so that the greatest caution should be exercised in drawing conclusions from the study of casts of the cranial cavity.”

Dura mater cast. The protrusions (h1, h2, *, *, *), observed in the superior ( Fig. 17 ) and lateral views ( Pl. III ) of the dura mater cast extend into corresponding invaginations of the intracranial surface ( Pl. IV ).

The dura mater cast enables us, however, to distinguish the actual exits of cranial nerves I, II, III, IV, V, VII, VIII, IX-XI, XII, which have been traced by Mr. Brown into openings which (with the exception of VIII) lead through to the outer surface of the cranium, as displayed in Figs. 7 , 8 . As above noted, VI has not been observed.

The cast indicates that the brain within was long and narrow. Successive swellings which are observed in the superior ( Fig. 17 ) and lateral ( Pl. III ) views indicate the location of the olfactory nerves, separated by the ethmoidal septum (f), of the olfactory lobes (Olf.), of the cerebral hemispheres (cer.), of the optic lobes (Opt), of the cerebellum (cbl.) and of the medulla oblongata.

Lower Jaws.

Text Figs. 18 , 19 , 20 , 21 .

The characters of the lower jaw of Tyrannosaurus are clearly presented in the inner and outer aspects of three specimens, Amer. Mus. Nos. 973 , 5027 , 5866 .

There are three remarkably characteristic features of these jaws.

1. The interdental outgrowths or expansions of the alveolar septa of the dentary constitute a row of rugose interdental plates or “ rugosae” in the lower jaw ( Figs. 18 , 20 , 21 ) corresponding with the interdental “ rugosae” of the maxillaries ( Figs. 6 , 23 ).

2. The broad prolongation of the splenials ( Fig. 18 , d. p.) which overlie the interdental rugosae. This appears to be a separate element which may be known as the supradentary.

3. Forward and downward extension of the articular, which appears to correspond with the prearticulars of Mosasaurs and other reptiles.

Dentary. The dentary proper is best displayed in jaw Amer. Mus. No. 973 ( Fig. 20 ); this contains alveoli for thirteen mandibular teeth. In Amer. Mus. No. 5027 there are fourteen teeth.

Between the alveoli ( Fig. 21 ) are thin alveolar septa which expand internally into the interdental “ rugosae.” Along the base of these rugosae runs a longitudinal groove (gr.) for the dental artery which gives off side branches (f.) to the lower portion of the teeth; between the thirteen teeth may be reckoned twelve more or less prominent interdental “ rugosae.”

The middle portion of the ramus is traversed by the horizontal Meckelian groove (Mk. gr.). The union with the opposite jaw was merely membranous, there being no indication of symphysial borders. The relations of the dentary, partly obscured by the overlying splenial and supradentary, is shown in Fig. 18 . The bone termed presplenial by Lambe in Albertosayrus is a part of the dentary, the supposed line separating it from the dentary below being the Meckelian groove (Gregory).

Supradentary. This is a dermal plate apparently of splenial origin but suturally separated from the splenial posteriorly ( Fig. 18 ) which extends forward (dp.), overlapping the basis of the teeth and dermal complex of the dentary. As shown by Williston the ‘ presplenial’ is the true splenial; the ‘ splenial’ of Baur is the true angular.1

Angular ( Figs. 1 , 18 ), with a limited exposure on inner side of the jaw and an extensive exposure on the outer side.

Surangular ( Figs. 1 , 18 , 19 ), with very extensive exposure on the outer side, forming the entire upper border of the inner side of the jaw above the pterygoid fossa; perforated by a fenestra (*, Fig. 1 ) or foramen (for., Fig. 18 ).

Coronoid ( Fig. 18 ), small, triangular, restricted to anterior border of pterygoid fossa.

Articular ( Figs. 1 , 18 , 19 ), forming the posterior portion of the ramus and broadly uniting on the upper surface ( Fig. 19 ) with the surangulars and supposed prearticulars. Relations to the quadrate are shown in Figs. 4 , 1 . The broad groove on the outer side of the articulars ( Fig. 19 hy) possibly served for the passage of the hyoid cartilage which in Sphenodon passes around the outer border of the articular, thence passing upward and inward (Gregory.)

TEETH OF Tyrannosaurus . Text Figs. 1 , 3 , 18 , 20-24 and Pl. I . The maxillary teeth are completely preserved in skull No. 5027 ( Pl. I) and in the maxillary fragment No. 973 ( Figs. 22 , 23 ). The mandibular teeth are shown in Nos. 5027 , 973 , and 5866 . The dental formulae are as follows: UPPER JAW: Premaxillary teeth, 4 Maxillary “ 12 total 16. LOWER JAW: Inferior maxillary teeth, 13

to 14. All the teeth in the upper jaw display the serrate edges characteristic of the Sauropoda, but the proportions of the crowns and of the external and internal or buccal and lingual surfaces, as defined by the positions of the serrate edges, are widely modified as we pass backwards from the extreme anterior to the extreme posterior members of the series. Superior Teeth.

The four premaxtllary teeth present a strong anterior convexity and lateral compression, the serrate edges being confined to the posterior lateral or lingual surfaces which are slightly convex. This type of tooth is adapted ( Pl.I ) for action.

The twelve maxillary teeth mark the graduated transition from the laterally compressed form with the serrate margins somewhat obliquely placed ( Pl. I ) to oval, lance-shape form, in which the teeth are laterally compressed and the serrated edges form the anterior and posterior borders of the crown. As the sections of the teeth change, the outer or buccal surface becomes reduced, the inner, or lingual surface is expanded until finally on the eighth maxillary tooth ( Pl. I ) the outer and inner surfaces of the crown are subequal. The first maxillary tooth is sharply demarcated from the lateral premaxillary tooth by its much greater size. The apparent irregularities of size in the adjoining teeth of the maxillary series are" due to the different degrees of the extrusion of the teeth. There is also ( Pl. I ) a gradual decrease in size from the first to the tenth maxillary tooth. The eleventh and twelfth maxillary teeth, however, are both of relatively diminutive size; the twelfth tooth is especially small.

Mandibular Teeth. The mandibular teeth vary numerically in two specimens from fourteen teeth in each side ( No. 5027 ) to thirteen teeth on each side ( No. 973 ). It is not known whether this is a matter of individual variation or of specific difference. As in the maxillary teeth, the outer surfaces are generally more convex than the inner surfaces. The crowns are gently concave on the inner surface near the base ( Fig. 21 ), which is perforated for a branch of the dental artery (/.). The successional teeth ( Fig. 24 ) arise on the inner side. There are always a double pair of small posterior teeth on each side which correspond with the single diminished teeth in the upper series.

The four anterior teeth of the mandibular series do not correspond in form and proportions with the four compressed premaxillary teeth in the upper series; only two, namely, the first and second mandibular teeth, are laterally compressed with deep sections and serrated edges on the posterior or lingual borders.

Of these two the second mandibular tooth is intermediate in form while the third is more like the teeth which follow.

The third to the thirteenth or fourteenth mandibular teeth are of sub-oval transverse sec­ tion; the anterior and posterior serrated edges are placed somewhat obliquely in the third to the fifth mandibular teeth; in the sixth to the thirteenth the serrated edges are placed directly on the anterior and posterior borders of the tooth so as to be observable in the lateral or outer view of the jaw ( Fig. 18 ).

There seems to be considerable variation in different individuals in the size of the mandibu­ lar teeth. In No. 973 the first mandibular tooth is much smaller than the second; in No. 5027 the first tooth while inferior in size to the second is a relatively larger tooth than the first, No. 973 .

Figures

Fig. 1. Tyrannosaurus rex. Key to Plate I. Abbreviations: ant. f', f", f'", first, second and third antorbital fenestrae. 'La.,’ “ lachrymal” (= adlachrymal Gaupp.) j. f., jugal fenestra. orb. rug., orbital rugosity. Op. o., opisthotic (paroccipital process). 4, fourth premaxillary tooth. 1, first maxillary tooth. *, fenestra in surangular,

Plate I. Complete skull of Tyrannosaurus rex Osborn. Amer. Mus. No. 5027. One-fifth natural size. The element shown in the upper posterior part of the first antorbital fenestra is the displaced ectopterygoid (transversum) of the opposite side.

Plate II. Tyrannosaurus rex. Superior view of skull. Amer. Mus. No. 5027. Scale about 1/6.

Fig. 2. Tyrannosaurus rex. Superior view of skull, Amer. Mus. No. 5027. Key to Plate II. Abbreviations as in preceding figures. sup. fen., superior temporal fenestra. lat. fen., lateral temporal fenestra. ‘La.,’ adlachrymal of Gaupp. P. o., postorbital + postfrontal. ‘Pr.fr.,’ lachrymal of Gaupp, prefrontal of authors.

Fig. 3. Tyrannosaurus rex. Full front and oblique front view of skull. Amer. Mus. No. 5027. Greatly reduced. ‘Pr.fr.,’ prefrontal (= lachrymal of mammals, fide Gaupp). ‘La.,’ “ lachrymal” of reptiles (= adlachrymal Gaupp).

Plate III. Tyrannosaurus rex. Section of skull showing brain cavity. Amer. Mus. No. 5029: Scale 1/2.

Fig. 4. Tyrannosaurus rex. Posterior view of skull. Amer. Mus. No. 5027. Ig. nu., pits for ligamentum nuchae. p. temp, fen., vestigial post-temporal fenestra. v. venous foramen. f. qu., quadrate foramen. d. p., presplenial process or supradentary. Pr. art., prearticular (dermarticular) .

Fig. 5. Comparison of the inferior aspect of the skull of Tyrannosaurus (A) and Dromceus (B). p. nar.j posterior nares. [Pa. sp.], position of parasphenoid. ' Ec. pt., ectopterygoid. The ectopterygoids (Ec. pt.) have slipped back from the normal contact with the posterior process of tbe maxilla, as indicated by the arrow.

Fig. 6. Tyrannosaurus rex. Inferior aspect of the skull, slightly corrected for distortion. Amer. Mus. No. 5027. Scale i. The ectopterygoids, Ec. pt., have slipped back from the normal connection with the maxillaries.

Fig. 7. Tyrannosaurus rex. Immature cranium with temporal arches removed, oblique view of left side. Amer. Mus. No. 5117. Scale 13/. ?P. sp., supposed presphenoid. Pa. sp., parasphenoid. ?, Marks supposed suture between ethmoid and presphenoid. Pr. f., prefrontal of authors (lachrymal of Gaupp). Pa. sp., parasphenoid. O. sp., orbitosphenoid (Gaupp), “ alisphenoid ” of authors. car. in., lower and upper entrance for internal carotid. fen. ov., fenestra ovalis (for stapes). i-vii, exits of cranial nerves i-vii

Fig. 14. Crocodilus sp. Longitudinal section of skull. Scale 4/9. 0. sp., orbitosphenoid (Gaupp), “alisphenoid” of authors. The sutural separation of the “epiotic” from the supraoccipital is doubtful. can. i. ty., canalis intertympanicus (median diverticulum of eustachian canal).

Fig. 15. Crocodilus sp. Half section of occiput. Scale 1/2. p. tern, fen., posttemporal fenestra. tu. cue., eustachian tube

Fig. 16. Diplodocus longus Marsh. Amer. Mus. No. 694. Scale 1/2. A, sagittal section with “ brain cast” in place. B, “ “ “ “ “ removed. S. end., saccus endolymphaticus. ves., region of vestibulum (recessus vestibuli).

Fig. 17. Tyrannosaurus rex. Superior aspect of brain cast, from skull no. 5029 Amer. Mus. (cf. Pl. III). Scale å. h‘, hi., membranous prolongations of dura mater. *, marks the position of the saccus vasculosus. †, marks the position of the ethmoid septum. Olf., olfactory peduncles. Op. l., optic lobes. V‘, Ramus ophthalmicus trigemini. V’, 3, Rami maxillaris et mandi- bularis trigemini.

Fig. 18. Tyrannosaurus rex. Inner view of left lower jaw, with the splenial and supradentary (d. p.) plate present. Amer. Mus. No. 5027. for., foramen in surangular. n.f., nutrient foramen. Mck. g., groove for Meckelian cartilage. d. p., supradentary plate (may be an outgrowth of the splenial).

Fig. 19. Tyrannosaurus rex. Superior view of articular region of lower jaw. Amer. Mus. No. 5027. hy., groove for hyoid bone.

Fig. 20. Tyrannosaurus rex. Amer. Mus. No. 973. Inner view of left dentary, with the splenial and supradentary absent. The small rugose plates between the teeth are outgrowths of the dentary alveolar borders. Below them is a groove for the bloofl vessel supplying the roots of the teeth (see Fig. 13).

Fig. 21. Tyrannosaurus rex. Inner view of dentary between exposed alveoli of 7th to 10th teeth. Amer. Mus. No. 973. Rela­ tions of the nutrient groove (gr.) and foramina (f) to the interdental plates (rugosae) and teeth .

Fig. 22. Tyrannosaurus rex. Left maxilla, outer view. Amer. Mus. No. 973. (Type.) Scale 1/6.

Fig. 23. Tyrannosaurus rex. Left maxilla, inner view of the same specimen, Amer. Mus. No. 973. Exhibiting eight of the maxillary teeth, four vacant alveoli, and nine interdental plates of the maxilla

Fig. 24. Tyrannosaurus rex. Cross section through the dentary showing a small successional tooth on the inner side of a. functional tooth. Amer. Mus. No. 973. Scale 1/2.

Fig. 26. Allosaurus agilis Marsh. Lateral view of cranium and lower jaw. Amer. Mus. No. 666. Scale 1/9. 1, 2, 3, antorbital fenestrze. La., laehrymal (adlaehrymal) rugosity. Pr.ƒ., prefrontals (separate, movable).

Fig. 8. Tyrannosaurus rex. Immature cranium, same as in Fig. 7, full front view. Amer. Mus. No. 5117. Scale 1/3.

Plate IV. Tyrannosaurus rex. Section of skull showing cast of brain in situ. Amer. Mus. No. 5029. Scale 1/2.