Cratis thylicus, P. G. Oliver & Holmes, 2004

Oliver, P. Graham & Holmes, Anna, 2004, Cryptic bivalves with descriptions of new species from the Rodrigues lagoon, Journal of Natural History 38 (23), pp. 3175-3227 : 3184-3186

Cratis thylicus n. sp.

( figures 37–43 )

HOLOTYPE: 1 lv from Grand Pate´ , off Port Mathurin, shell gravel, 17 m,

19 ‡ 39.307 ’ S, 63 ‡ 24.950 ’ E. NMW.Z.2001.061.00014.

PARATYPES: 5 lv and 2 rv as holotype, NMW .Z.2001.061.00015.

Measurements (mm, L 6 H). Holotype: 1.34 6 1.40. Paratypes: 1.35 6 1.53, 1.37 6 1.61, 1.07 6 1.23, 1.28 6 1.53, 0.97 6 1.22, 1.32 6 1.52, 1.04 6 1.34.

Description. Size 1.6 mm. Equivalve. Inequilateral; umbo situated on anterior margin. Outline obliquely oval trigonal; dorsal margin straight and short, sloping slightly to the anterior; anterior margin straight, sloping to the ventral margin; ventral margin rounded; posterior margin slightly concave with a very large cavity along most of posterior margin. The cavity ( figures 41, 43 ) is fairly deep and is 160 M m wide without the periostracum. The periostracum extends further to the posterior to encase the cavity ( figure 39 ). Hinge bears a set of provincular denticles to the posterior of the ligament ( figure 40 ) and both valves have a single, large prionodont tooth ( figure 40 ) and associated socket. At and just below the posterior end of the provincular denticles is a small tubercle probably representing a posterior prionodont tooth. Ligament pit small, shallow, trigonal and slightly prosoclinal.

Sculpture consists of equally spaced, fine, concentric ridges. Periostracum lamellar, arising from the concentric ridges except for about six radial rows of adpressed spines which project beyond the ventral margin. Prodissoconch (PdI) a simple, shallow cap, about 230 M m width, with faint concentric lines on the outer edges ( figure 40 ). Inner margin with two or three crenulations at the posterior ventral junction. The shell is translucent although the concentric lines, myostracum, edge of posterior margin and periostracum are opaque.

Derivation of name. Greek thylakos ‘a pouch’, referring to the posterior cavity.

Comparisons. We have reviewed all of those species attributed to Cratis and those that resemble the genus including Cosa and Nipponolimopsis and could find none with a posterior cavity. All of those taxa from New Zealand and Australia have capped prodissoconchs with a median boss quite unlike the low, rather featureless prodissoconch of the Rodrigues species.

Remarks. The function of the posterior cavity is not readily apparent as no living animals were collected. From observations on other philobryids many brood their young ( Tevesz, 1977; Hayami and Kase, 1993). C. thylicus has a large but simple prodissoconch and is also likely to brood its young and we suggest that the cavity acts as an external marsupium for this purpose.

Figures

FIGS. 37–43. Cratis thylicus n. sp. (37–39) Holotype, NMW.Z.2001.061.00014, left valve: (37) external (MontageTM); (38) external (SEM); (39) internal (MontageTM). (40–43) Paratype, NMW.Z.2001.061.00015, left valve, periostracum lacking: (40) hinge and prodissoconch (SEM); (41) internal (SEM); (42) external (MontageTM); (43) internal (MontageTM).