Stockumites exilis (Vöhringer, 1960) comb. nov.

Figs 46–47; Tables 40–41

Imitoceras liratum exile Vöhringer, 1960: 126, pl. 2 fig. 3, text-fig. 6.

Acutimitoceras exile – Korn 1994: 45, text-figs 46c–d, 50h, 52d, 54b–c.

Diagnosis

Species of Stockumites with a conch reaching 40 mm diameter. Conch at 4 mm dm thinly pachyconic, subinvolute (ww/dm ~0.65; uw/dm ~0.15); at 12 mm dm thickly discoidal, involute (ww/dm ~0.55; uw/dm ~0.05); at 20 mm dm thickly discoidal, involute (ww/dm ~0.55; uw/dm ~0.03). Whorl profile at 20 mm dm weakly compressed (ww/wh ~0.85); coiling rate moderately high (WER ~1.80). Venter rounded, umbilical margin rounded. Growth lines fine, wide-standing, with convex course. Moderately deep constrictions on the shell surface; weak internal shell thickenings. Suture line with lanceolate external lobe and V-shaped adventive lobe.

Material examined

Holotype

GERMANY • Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 4; Vöhringer Coll .; illustrated by Vöhringer (1960, pl. 2 fig. 3) and Korn (1994, text-fig. 46d); re-illustrated here in Fig. 46A; GPIT-PV-63868.

Paratypes

GERMANY • 2 specimens; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 3b; Vöhringer Coll.; GPIT-PV-63870, GPIT-PV-64019 • 2 specimens; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 3c; Vöhringer Coll.; GPIT- PV-64018, GPIT-PV-64021 • 1 specimen; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 3d; Vöhringer Coll.; GPIT-PV-63933 .

Description

Holotype GPIT-PV-63868 is a moderately well-preserved specimen with 22 mm conch diameter (Fig. 46A). It has a discoidal shape (ww/dm = 0.45) with a closed umbilicus, a compressed whorl profile (wh/dm = 0.85) and a low coiling rate (WER = 1.73). The shell surface is preserved on the last half whorl and shows fine, widely spaced growth lines. These extend almost straight across the flank and bend backwards in the ventrolateral area to form a ventral sinus. Parallel to the growth lines, distinct shell constrictions are arranged at intervals of less than 90 degrees (Fig. 47D). These constrictions vary in depth.

The suture line of paratype GPIT-PV-64018 has a V-shaped external lobe, a rather narrow, narrowly rounded ventrolateral saddle and a V-shaped adventive lobe (Fig. 47B). The latter is slightly wider than the external lobe and has slightly convexly curved flanks.

Remarks

Stockumites exilis belongs to the species of the genus with a small conch. The species is distinguished from most other species by its very slender conch. The combination of these features with the distinct shell constrictions, which are absent in many of the species of Stockumites, is important to characterise this species.