Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 72. (Budapest 1980)

Szabó, J.: Lower and Middle Jurassic Gastropods from the Bakony Mountains (Hungary). Part II.: Pleurotomariacea and Fissurellacea (Archaeogastropoda)

other hand, these latter may cover sometimes the whole surface, but in this case these are visible only under high magnification. The run of the growth-lines is somewhat forwardly-arched prosocline above the selenizone, opisthocline below the selenizone, and reversed "S"-shaped on the base, respectively. Embryonal shape and ornament —The protoconch and the first whorl is slightly depressed, but the adult spire angle appears still from the third whorl onwards. The protoconch and the first half whorl is depressed, smooth, then the selenizone appears, and the shell becomes ornamented with cancellate sculpture, simultaneously. Remarks — The juvenile shell, up to the fifth whorl, corresponds well to that in the genus Pyrgotrochus, the whorl-surface being flattened from the second whorl, the selenizone situating near to the periphery appearing as a strongly swollen band. On the other hand, the selenizone tends to become more elevated from the sixth whorl, and the band above the suture becomes smooth. The whorl-section of the adult shell is similar to that in the high-spired Bathrotomaria. Similar tendencies were recorded in the shell-building of Pyrgotrochus periferialis sp. n. (see above). Distribution — Bakony Mts., Somhegy, Humphriesianum to Parkinsoni Zones. Cyclostomaria gen. n. Type species: Cyclostomaria suessi (Hörnes,1853). Derivatio nominis: Referring to the circular peristome. Diagnosis : The whorl-section is circular; the selenizone is wide, running near the periphery; the periphery is without angulation; the spire is low; the suture is situated in a deep groove; the umbilicus is wide, open; the growth-lines are prosocline, somewhat prosocyrt between the suture and the selenizone, strongly prosocyrt below the selenizone, and prosocline, generally sigmoidal on the base; the ornament consists of gowth-lines bounding into ridges or flattened nodes, spiral threads, cords and grooves. Remarks — Similar forms are represented within the low-spired Leptomaria and Bathrotomaria, as well as within the hardly ornamented Obornella species, however, these show angular periphery and flattened whorl-surface. Distribution — Lower and Middle Jurassic [e.g. C. semiornata (STOL.-LAUBE, 1868)], "Pleurotomaria" spilsbyensis Cox 1960; "PI" anstedi FORBES, 1845). These two latter species staged a problem for Cox H960) in the generic arrangement. Cyclostomaria suessii (HÖRNES, 1853) (Plate IV: fig. 4.) 1853: Pleurotomaria Suessii HÖRNES — HÖRNES, in Hauer, p. 762. 1861: Pleurotomaria Suessii HÖRNES — STOLICZKA, p. 192, pi. 5, fig. 1. 1911 : Pleurotomaria Suessii HÖRNES — M. GEMMEÍÍLLARO, p. 213, pl. 10, figs. 10-12. Measurements: H HL HA W D A Plate IV: fig. 4 20.5 18.5 19 46 — Material — Five specimens. Shape — Dextral, nearly discoidal form with depressed spire. The surface of the whorls is convex, the whorl-section is subcircular. The suture runs in a deep groove. The selenizone is rather wide, concave initially, becoming convex on the last whorl. The periphery is rounded, the base is convex, and rounds into the extremely wide, open umbilicus after a flattened, longitudinal swelling. The peristome is unknown. Ornament — The surface of the whorls is covered with spiral cords, which are interrupted on the selenizone, and are intersected by transverse cords running down to the periphery. These ornamental elements become flattened and fade out on the last whorl. The base is ornamented with some indistinct spiral lines and conspicous growth-lines. The umbilical region is similarly ornamented, however, here the growth-lines bound into ridges Remarks — The spire angle of the available specimens is bigger than that of the forms figured by STOLICZKA, but the ornament is similar. On the other hand, the description shows, that the Alpine material yielded some nearly discoidal specimens, too. This suggests, that this is a highly variable species. Presumably the spire angle becomes bigger in the stratigraphically younger forms, i.e. the certainly Upper Pliensbachian examples show depressed spire. Similar is the Pleurotomaria platyspira of E. DESLONGCHAMPS (1849), but it is costate between the selenizone and the suture — a feature seen in the here described form only on the juvenile shell.