Spirancilla milthersii ( Ravn, 1902 )

Hansen, Thomas, 2019, Gastropods from the Cretaceous-Palaeogene boundary in Denmark, Zootaxa 4654 (1), pp. 1-196 : 144-145

Spirancilla milthersii ( Ravn, 1902)

Figs 31 Q–R

1902 Ancilla Milthersii n. sp. Ravn: 234–235, pl. II, fig. 18.

Diagnosis. Protoconch conical with just over two convex whorls. Teleoconch shell elongated with weakly ex- pressed sutures and no spire furrows. Columella almost vertical, bearing four to six narrow, oblique plaits. Parietal callus very thin except in front of adapical part of aperture, forming weak tongue adapically of aperture. Aperture taking up slightly over half the shell height; its outer lip lacking a labral denticle.

Material. Holotype MGUH 33270 (cast), MGUH 33271 , a mould with the informal sample number SH.192. A–B, and two uncatalogued specimens deposited in the collections of the Natural History Museum, Copenhagen.

Type stratum and type locality. The single specimen of Ravn (1902), MGUH 33270, and therefore the holotype, is from the Cerithium Limestone Member at Stevns Klint. The last known location for the specimen was in the collections of DGU [= Danmarks Geologiske Undersøgelse], which is now part of the semi-private institution GEUS in Denmark. In spite of several attempts at contacting GEUS for more information on its whereabouts, no information has come to hand, and the specimen may very well have become lost.

Occurrence. Rare in the Cerithium Limestone Member at Stevns Klint.

Description. Shell small, narrowly ovate with bluntly pointed apex. Protoconch low conical with just over two convex whorls, about 0.45 mm high and 0.65 mm wide. Teleoconch spire weakly convex with flat whorls covered by thin callus obscuring sutures. Last whorl contributing around 78 % of shell height, moderately convex, evenly contracted anteriorly, with broad uncallused zone between abapical edge of spire callus and posterior fasciole.Aperture elongate-ovate, strongly notched anteriorly, taking up about 54 % of shell height. Outer lip with evenly curved anterior margin lacking labral denticle. Columella thickened, gently concave, with four to six narrow plaits continuing into the aperture. Columellar callus bordered adapically by shallow anterior fasciole furrow, furrow terminating at aperture. Middle fasciole slightly wider, terminating just adapical to columella, and divided from the former by more or less distinct groove. Posterior fasciole widest, often indistinct due to effacement of adapical groove, groove terminating at adapical aperture angle. Spire grooves, including ancillid groove and band absent. Parietal callus very thin but for weak, rather narrow and nearly straight-sided tongue reaching up from aperture to spire callus, widening just before reaching the latter.

Measurements. Holotype MGUH 33270 is 3.3 mm wide and 7.8 mm high, composed of protoconch and about four teleoconch whorls.

Remarks. Ravn (1902) did not explain the reason for the taxon name, but it was clearly in honour of the geologist and collector of the specimen, Poul Christian Vilhelm Madsen Milthers, who found it in the year 1900.

In spite of what was indicated by Vermeij (2001: p. 508) Spirancilla milthersii does not posses a labral denticle, but has an evenly curved anterior outer lip margin.

Spirancilla milthersii ( Ravn, 1902) resembles the Selandien Spirancilla flexuosa ( von Koenen, 1885) from Denmark, but it appears smaller with a more blunt spire, a deeper anterior fasciole furrow and with stronger columellar plaits.

Figures

FIGURE 31. A–C. Vexillum tenestolidum n. sp. A–B. Silicone cast of holotype MGUH 33259. Width of protoconch 0.7 mm. Width of last whorl 3.4 mm. C. Silicone cast of paratype MGUH 33260. Width 5.8 mm. D–G. Vexillum? rosenkrantzi n. sp. D–E. Silicone cast and inner mould of holotype MGUH 33261. Width 1.9 and 1.6 mm respectively. F. Silicone cast of paratype MGUH 33262. Width 3.2 mm. G. Silicone cast of paratype MGUH 33263. Width 2.9 mm. H. Vexillum? sp. Silicone cast of MGUH 33264. Width 3.5 mm. I. Columbarium heberti (Briart & Cornet, 1880). Silicone cast of last whorl MGUH 33265. Width 7.7 mm. Photograph by S.L. Jakobsen. J. Columbarium sp. Silicone cast of MGUH 33266. Width 13.3 mm. K–N. Conomitra glabra (Ravn, 1902). K. Silicone cast of MGUH 33267. Width 3.6 mm. L–M. Silicone cast of MGUH 33268. Width 3.6 and 3.9 mm respectively. N. External cast of syntype MGUH 116. Height 8.2 mm. Photograph by S.L. Jakobsen. O. Volutomitra cf. quinqueplicata Ravn, 1902. Silicone cast of MGUH 33269. Width 11.6 mm. P. Volutomitra quinqueplicata Ravn, 1902. External cast representing last whorl of type specimen MGUH 117. Middle Danien Faxe Formation, Denmark. Height 38.6 mm. Photograph by S.L. Jakobsen. Q–R. Spirancilla milthersii (Ravn, 1902). Q. Silicone cast of holotype MGUH 33270. Width 3.3 mm. R. Silicone cast of MGUH 33271. Width 3.1 mm. S–T. Exilia sp., Silicone cast of MGUH 33272. Width 1.6 mm. U–W. Latiromitra? sp. External cast and inner mould of MGUH 33273. Width 4.6 mm for casts and 4.0 mm for mould.