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)

facies, too, suggesting preferred biotopes on the deeper and quiter water parts of the submarine topographic highs. Stratigraphie value — Gastropods are extremely rare in the Mediterranean Jurassic. Their approximate ratio of the other fossils with shell is bivalve 0.1, gastropod 1, ammonite 50, brachiopod 100, specimens for the Hierlatz limestone, one of the facies richest in gastropods. This sparsity makes relatively hopeless to find index fossils among them. As it is clear from Fig. 3., the vertical distribution of the more common Discohelix and Pentagonodiscus extends to about a stage. The rarer species are represented in single occurrences, thus, their evaluation is difficult. To clear up their evolution needs more data on greater material, possibly from additional localities. Systematic descriptions Subclass PROSOBRANCHIA MILNE EDWARDS, 1848 Order ARCHAEOGASTROPODA THIELE, 1925 Superfamily Euomphalacea DE KÖNINCK, 1881 Family Euomphalidae DE KÖNINCK, 1881 Genus DISCOHELIX DUNKER, 1848 Discohelix orbis (REUSS, 1852) (Plate I: figs. 1-3; Fig. 6e) 1852: Euomphalus orbis REUSS-REUSS, p. 114., pi. 14., fig. 1. 1861: Discohelix orbis REUSS-STOLICZKA, p. 182., pi. 3., figs. 8-10. 1874: Discohelix orbis Reuss-Gammellaro, G. G., p. 98. 1912: Discohelix orbis REUSS-HAAS, p. 283., pi. 20., figs. 23-24. 1924: Discohelix orbis Reuss-Maugeri, p. 48., pl. 1., fig. 26. 1937: Discohelix orbis REUSS-PŐELINCEV, p. 26., pl. 1., fig. 41. Measurements : I) H S H/D H/S A U Plate I: fig. 1 31 9 7.2 0.29 1.25 210° 150° Plate I: fig. 2 14.5 4.8 3.7 0.33 1.30 208° 142° Plate I: fig. 3 25.1 6.5 5 0.26 1.20 — 151° Material — Eighty-eight specimens coming from almost all localities. More than half of the material consist of internal moulds with damaged or fragmentary shell. Other specimens are internal casts or impressions. Two-thirds of the internal casts are inner, one-third is outer moulds. Shape — Dextral, discoidal form with concave spire. From the spiral plane of the nearly bilaterally symmetrical shell only the embryonal part raises slightly on the spiral side. The whorlsection is trapezoidal, with slightly convex sides. The carinate keels are accenuate throughout the whole shell. The peristome is simple, entire, sometimes is slightly broadened in every direction. Sculpture — About 2 whorls after the disappearing embryonal sculpture the spiral ornament reappears. It consists of fine lines occurring on the spiral, umbilical and outer sides. Growthlines are visible slightly earlier. In adult stages weak, transversal ridges appear more or less periodically on the spiral and umbilicial sides. With the onset of the visible growth-lines, the carinae of the keels become corrugated. This corrugation endures up to the peristome. Embryonal shape and sculpture — The coiling of the embryonal shell begins differently from that of the adult shell, i.e. the nucleus and about a half whorl are slightly raised on the spiral side. Approximately a half whorl after the nucleus, spiral lines appear on the shell which are visible on about two whorls, then fade away. There is no sharp boundary between the embryonal and the adult shell. Remarks — The relatively large material made possible to study the variability of this species. Because of the rather stable ornamentation, the variability is better reflected by the dimensions (Fig. 5). The diagram is based 50 specimens with measurable height and the related diameter and whorl-width. The points on the figure represent specimens, on the basis of the two ratios. The figure shows both the relation to the other Disco/ielix species and the difference between the variability