Diarthema Piette, 1864

Szabó, János & Jaitly, Anand Kumar, 2019, Additional Middle Jurassic gastropods from Kachchh (western India) in the collections of the Banaras Hindu University (Varanasi, India), Fragmenta Palaeontologica Hungarica 36, pp. 3-30 : 23-24

Genus Diarthema Piette, 1864

Type species – Pterocera paradoxa J. A. Eudes-Deslongchamps, 1843

Diarthema sp.

( Figs 67–74)

Material – Two deformed specimens, both are adaxially compressed inner moulds, bearing also badly worn shell fragments.

Measurements – Inv. No.: BHU 2020I 306 ( Figs 67–70); H dam. = 15.3 mm; Inv. No.: BHU 2020I 307 ( Figs 71–74); H dam. = 14.2 mm.

Description – Moderately high, few-whorled (~5), pagodiform shells with low spire and blunt apex. The earliest whorl (= protoconch?) seems nearly planispiral then the earliest teleoconch whorl has rounded surface, and about three whorls of the teleoconch with marked spiral angulation follow it. A rather wide, abapically-abaxially, mildly sloping ramp and an abapically-adaxially sloping outer face compose the whorl surface. Both the ramp and the outer face are just slightly convex or flat. For the last whorl, the angulation vanishes and the slightly convex whorl surface passes to the highly conoidal base without distinct limit. Suture is well impressed. Remnants of the latest inner lip indicate an obliquely elongated peristomal complex. Its abapical part extends left well beyond the axis and has the shape of a siphonal notch. The adapical part extends up to the ramp/ upper suture area on the initial last whorl and probably also to the outer face of the penultimate whorl as some remnants indicate it. The inner lip between the two terminal parts appears as rather thick callus on the base.

Ornament is visible on the last three whorls of both specimens as sparse spiral cords of unequal thickness and dense, transverse riblets, which have been hollow internally or the shell itself has been substituted by sediment; the riblets become gradually stronger (ribs); they extend from suture to suture. Together with this ornament, the angulation of the whorls also developed. The spiral ornament elements last to the peristome as sparse and sharp cords and keels but, after reaching their strongest form on the penultimate whorl, the periodically repeated ribs are already lacking. On the last whorl, only few varices are the transverse ornament, showing the places of former, not periodically grown, extended peristomes.

Remarks – The two specimens significantly differ from each other; they seem to belong to a highly variable species. However, the specimen in Figs 71–74 resembles also Struthiolaria and the poorly preserved sharp ridge that seems to be the edge of a former outer lip, shown in Fig. 73, supports its place in Diarthema but perhaps in different species. Well-preserved material is needed to a more accurate identification. No species has yet been found in the literature that would show close similarity to these specimens.

Occurrence – Habo Dome, Pacham Formation, Bathonian.