Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 71. (Budapest 1979)

Szabó, J.: Lower and Middle Jurassic Gastropods from the Bakony Mountains (Hungary). Part I.: Euomphalidae (Archaeogastropoda)

Embryonal shape and sculpture — The smooth, globular nucleus and slightly the first whorl too, are raised on the spiral side. About a half whorl after the nucleus, 3 to 4 fine spiral lines appear, which endure through 1 to 1.5 whorl, and then disappear. Remarks — The available specimens agree well with the descriptions and figures given by WENDT (1968). However, in the Bakony Mts. material the markedly sculptured forms shown in his pi. 110., figs. 9-12. are absent. — According to the present writer's judgement, the figured Sicilian and Bakony Mts. forms cannot be identified with D. cotswoldiae (LYCETT, 1850). Distribution — Western Sicily, Aalenian — Lower Bajocian; Bakony Mountains: Somhegy, Humphriesianum to Parkinsoni Zones. Genus PENTAGONODISCUS WENDT, 1968 Euomphalid species with pentagonal shape are known from the Lower Liassic onwards. Earliest representative is the Pentagonodiscus mellonii (GEMMELLARO, 1879), which still shows trochospiral coiling. Other species (P. reussii, P. initiopentagonatus sp. n., P. angustus) have biconcave shell. The shape is extremely characteristic; the pentagonal symmetry, as traces in the sculpture, remains even when the normal logaritmic spiral coiling is partly restored (e.g. P. initiopentagonatus sp. n.). This feature makes these forms easily distinguishable from the Discohelix species. This morphological difference, together with the wide distribution, gives reason to separate it as an independent genus. Pentagonodiscus reussii (HÖRNES, 1853) (Plate II: figs. 4-5; Fig. 6g) 1853: Euomphalus reussii HÖRNES — in HAUER, p. 760. 1861: Discohelix réussi HÖRNES-STOLICZKA, p. 184., pl. 3., fig. 14. 1911: Discohelix réussi HÖRNES-M. GEMMELLARO, p. 215., pi. 9., fig. 14. 1968: Discohelix (Pentagonodiscus

) reussii (HÖRNES, 1853) — WENDT, p. 574., pi. 110., figs. 22-24, text-figs. 2G., 3P., 5. Measurements: D H S H/D H/S A U Plate II: figs. 4-5 10.7 4.8 2.6 0.45 1.85 204° 118° Material — Ten internal moulds, with more or less fragmentary, strongly recrystallized test. Shape — Dextral form, the spire is less concave than the umbilicus. In axial view the shell — except the embryonal whorls — is pentagonal. The whorl-section on the spiral and umbilical side is slightly, on the outer side is strongly convex. The peristome is narrowed by the keel-to-keel inner varix. The inner lip is also somewhat thickened by a thin plate. The inner varices are repeated periodically at intervals of somewhat less than 72°, their places are marked on the surface of the whorls by transverse swellings on the spiral and umbilical sides and shallow successive depressions on the outer side. The inner varix marks an adaxial narrowing of the whorl, while the outer broadening shows an axial expansion •— thus the area of the whorl section remains unchanged. The outer swellings, with their constant < 72° periodicity resemble some posteriorly arched spiral arms. Sculpture — The adult sculpture onsets at about the end of the third whorl. The recurring spiral and the transversal sculpture, as well as the pentagonal shape appears roughly simultaneously. The spiral sculpture consists of threads on the whole whorl-surface, which are separated by narrow incisions. The transversal sculpture is resulted from costellae reaching from suture to the keels. These costellae become stronger toward the keels, corrugating the latter, and then rapidly fade out outward. The spiral and transversal sculpture results in a retiform ornamentation. Embryonal shape and sculpture — The embryonal whorls show a normal logaritmic spiral in coiling, with slight elevation on the spiral side of the nucleus and approx. one whorl. About a half whorl after the nucleus appears a spiral sculpture of 3 to 4 fine lines, which disappears after about two whorls, and renews again on the adult shell. Remarks — One of the available specimens shows a „parabolic notch" on the outer side, following the inner varix (Plate II: fig. 5). It is problematic to distinguish the juvenile specimens of P. reussii and P. initiopentagonatus sp. n. (see below).