Streptochetus, Cossmann, 1889

Hansen, Thomas, 2019, Gastropods from the Cretaceous-Palaeogene boundary in Denmark, Zootaxa 4654 (1), pp. 1-196 : 136-137

Streptochetus sp. A

Figs 30 N–O

Material. A nearly complete external mould, MGUH 33253, and two incomplete moulds from the uncatalogued old collections at the Natural History Museum of Denmark. The latter two are collected by A. Rosenkrantz at Kirkebraade (19.091927) and Skeldervig (August 1944).

Occurrence. Very rare in the Cerithium Limestone Member , where it has been found at Skeldervig and Højerup, Stevns Klint.

Description. Protoconch conical with between 1 ½ and 2 smooth, convex whorls separated by deep suture; protoconch measuring 0.55 mm in width and 0.35 mm in height.

Teleoconch spire slender with convex whorls separated by moderately deep suture; whorl height less than half the width. Aperture narrowly ovoid with concave, seemingly smooth columella and oblique and wide siphonal canal; height of aperture and siphonal canal corresponding to 45 % of shell height. Siphonal canal short, only half as long as aperture. Fissure or weak pseudoumbilicus present. Siphonal canal with rounded distal end.

Teleoconch sculpture dominated by around eight moderately strong, convex and more or less orthogonal transverse ribs per whorl. Transverse ribs crossed by six fairly broad and flat spiral cords between sutures; transverse ribs highest at periphery, fading out toward sutures; not organized in axial rows. Spiral cords strongest at whorl periphery. Growth lines slightly sigmoid.

Measurements. MGUH 33253 is 9.2 mm high and approximately 3.6 mm wide, consisting of protoconch and six teleoconch whorls.

Remarks. It is not possible to verify whether this taxon carries teeth on the outer lip or not. According to Wenz (1943: p. 1264) the genus should be characterized by a slightly oblique protoconch and a sharply cut distal end of the siphonal canal. Neither feature is observed in the present taxon, which otherwise is well in accord with the diagnostic features of this genus.

Figures

FIGURE 30. A–B. Amaea sp.A. Details of protoconch on external mould of MGUH 33242 shown in natural view and mirrored with inverted colours. Width of protoconch 0.6 mm. C–D. Amaea sp. B. C. Silicone cast of protoconch MGUH 33243. Width 0.6 mm. D. Silicone cast of protoconch MGUH 33244. Width 0.4 mm. E. Amaea? sp. C, Silicone cast of ØSM.10042-373. Width 5.5 mm. F–G. Acirsa torensis n. sp. F. Silicone cast of holotype MGUH 33245. Width 0.8 mm. G. Silicone cast of paratype MGUH 33246. Width 1.1 mm. H. Acirsa sp., Silicone cast of MGUH 33247. Height 6.2 mm. I–K. Opaliopsis carlsbergi n. sp. I. Silicone cast of holotype MGUH 33248. Height 2.2 mm. J. Silicone cast of paratype MGUH 33249. Height 1.7 mm. K. Silicone cast of paratype MGUH 33250. Height 3.0 mm. L–M. Fusinus schnetleri n. sp. L. Silicone cast of holotype MGUH 33251. Height 23.9 mm. M. Silicone cast of paratype MGUH 33252. Height 13.4 mm. N–O. Streptochetus sp. A. Silicone cast of MGUH 33253. Width of figure N 2.7 mm. Width of teleoconch 3.5 mm. P–Q. Streptochetus? sp. B. Silicone cast of MGUH 33254. Teleoconch width 2.6 mm.Width of protoconch 0.6 mm. R–T. Latirulus? sp. R–S. Silicone cast of MGUH 33255. Width of protoconch 0.7 mm. Height of teleoconch 10.5 mm. T. Silicone cast of MGUH 33256. Width 3.7 mm. U–W. Dolicholatirus lorenzi n. sp. U. Silicone cast of holotype MGUH 33257. Height 30.8 mm. V–W. Silicone cast of paratype MGUH 33258. Width of teleoconch 5.5 mm. Width of protoconch 0.7 mm. X. Vexillum tenestolidum n. sp., Silicone cast of holotype MGUH 33259. Width of teleoconch 4.3 mm.