Acirsa, Morch. This, 1857

Hansen, Thomas, 2019, Gastropods from the Cretaceous-Palaeogene boundary in Denmark, Zootaxa 4654 (1), pp. 1-196 : 132-134

Acirsa sp.

Fig. 30H

Material. Specimen MGUH 33247 is an external mould consisting of seven teleoconch whorls.

Occurrence. Lithified top of the Højerup Member of the Maastrichtian Tor Formation at Mandehoved, Stevns Klint.

Description. Protoconch not preserved. Teleoconch whorls very weakly convex, twice as wide as high and separated by deep linear suture located immediately abapically to whorl periphery. Transition to convex base rather gradual. Aperture unknown. Teleoconch sculpture consisting of around 13 sharp ridged, low, axially oriented transverse ribs; ribs tapering out at transition to base. Ribs crossed by fine, orthocline to slightly prosocline growth lines and up to around 15 weak and densely spaced spiral striae. Periphery with coarser spiral furrow, marking transition to coarser, scattered spiral striae on base.

Measurements. MGUH 33247 is just over 2.0 mm wide with a preserved height of 6.2 mm.

Remarks. Acirsa sp. differs from Acirsa torensis n. sp. described above by the apparently lower number of transverse ribs, which are less opisthocline in nature, the less convex whorls and the seemingly more slender spire. On the other hand these differences could merely reflect ontogenetic changes as Acirsa sp. is represented by a single teleoconch, while the material of A. torensis n. sp. is limited to two juvenile specimens. Acirsa sp. differs from the Early Cretaceous A. miyakoensis ( Nagao, 1934) and A. ofunatoensis Kase, 1984 from Japan by the finer transverse ribs and less sharp transition to the base. It appears close to the Late Cretaceous Acirsa culmosa Sohl, 1964b from the USA, but differs by the slightly less convex whorls with apparently a much less dense sculpture of spiral striae on the base and a complete termination of the transverse ribs immediately abapically of the periphery. It is characterized from the slightly younger Selandian Acirsa elatior ( von Koenen, 1885) from Denmark by a more slender spire and less convex whorls.

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.