Szabó János szerk.: Fragmenta Mineralogica Et Palaentologica 20. 2002. (Budapest, 2002)

66 SZABÓ, been also partly mineralised (see Plate I: 10). On the outer (back) side, where the growth-lines are opisthocyrt, the axes of the fibres are oriented towards the periphery and resemble to being "focused" by a concave mirror to the outermost shell line. The lower layer, exposed on the umbilical side near the apparent peristome, show a French-floor pattern in section parallel to the shell surface (horizontally) but prismatic vertically. The prisms slope forward/towards the peristome, and the measurable angles suggest they are asymmetrical (columnar) first order calcite rhomboèdres in mineralogical sense. Remarks — The interspace between the adapical pair of carinae resembles to the sinus belt ("selenizone") of some euomphalid and ophiletid genera. The discoidal species of similar age, that mainly belong to Discohelix, are easily distinguishable by the genus characters. Comparisons and conclusion Similar gastropods to Valamidiscus n. gen. can be found mainly amongst the Early Palaeozoic representatives of the Macluritina [sensu KNIGHT et al. (I960)] or Euomphalina DE KONLNCK, 1881 [as WAGNER (2002) calls practically the same group of lower level taxa]. False sinistral shells with concave spire, loose coiling mode, transformations of trapezoidal whorl cross-section and (nearly) flat base are characteristic to the members of the Macluritidae (e.g. Maclurina ULRICH & SCOFLELD, 1897) and Ophiletidae (e.g. Lecanospira BUTTS, 1926 or Lesueurilla KOKEN, 1898). The spiral belt between the closely spaced adapical carinae in Valamidiscus n. gen., resembling to a sinus belt ("selenizone"), increases the similarity to the ophiletids. However, none of the compared forms have opisthocyrt growth-lines on the outer face. On the contrary, they have prosocyrt growth-lines. Further genera with concave spire and two angulations on the whorls are known also in the Euomphalidae, sensu BÄNDEL & FRYDA (1998) who suggested subclass status for Euomphalus SOWERBY, 1814 and other closely related genera on the basis of their peculiar protoconch morphology. They have a bulb-shaped initial chamber and at least half a whorl is open coiled. Though the remnants of the protoconch in the available specimen of Valamidiscus n. gen. are comparable, the poorly preserved earliest whorls do not permit a reliable decision if it has belonged to the Euomphalidae of this latter definition or not. The stmcture of the shell is different from that observed by BANDEL & FRYDA (1998) in Euomphalus. The third carina and the asymmetry in the growth-lines of the spire and umbilical sides in Valamidiscus n. gen. distinguish it from Discohelix DUNKER, 1848, the most common, contemporaneous discoidal genus. The unique shell structure may mean a further tool to the distinction, because MORRIS & CLEEVELY (1981) and SCHRÖDER (1995) found Discohelix to have prismatic nacreous inner shell layer. However, the figured gastropod of the latter author is so different from the type species of Discohelix (D. calculiformis), that a confirmation is needed from study of a species, being morphologically closer. MORRIS & CLEEVELY (1981) did not give either a figure or species name of their Discohelix species. The nacreous shell microstructure and BANDEL (1988) suggestion, induced by the "archaeogastropod type" protoconch, lead to establishing of Discohelicidae SCHRODER, 1995. The new systematical place would be the Trochoidea, however, this is a foreign morphological environment. Later BANDEL & GELDMACHER (1996) thought the cirroidean origin also possible. In Cirroidea, discoidal forms have been also accommodated and ultradextral (false left-handed) forms are common. Therefore tFiis sur^rfamily seems also a systematical place for Valamidiscus n. gen. to be considered. Fa spite of the doubts, Discohelicidae seems the best tentative systematical position on the family level for Valamidiscus n. gen., because a peculiar morphological element, namely the "spiral arm" pattern, has been observed in a Discohelix species (D. acarinata SZABÓ, 1979). This character may indicate close relation. Outer calcite shell layer in the compared gastropod genera and family level taxa is common but inner, originally calcite layer has not yet been reported. This magnitude of difference in the shell stmcture is frequently applied to distinguish families or even higher taxa. Unfortunately, the number of existing data is ^sufficient to understand how the mineralogy and microstmcture of the shells has changed and how it correlates with phylogeny/evolution. Estimation of the real value of this character is especially hard in the euomphalus-shaped gastropods that are known mainly from the early (PalaeozoicMesozoic) history of Gastropoda and belonging to extinct groups. Therefore the single occurrence of Valamidiscus n. gen. suggests treating of this case cautiously. In most of the existing gastropod classifications, the lower rank taxa in a subclass are different in their shell structure and in the protoconch morphology. These reflect a widely accepted opinion that the evolution in these character groups "has not stopped at the taxonomical boundaries". In fact, establishment of Subclass Euomphalomorpha BANDEL & FRYDA, 1998 provides a new theoretical ground to unify again those morphologically related genera that have got into uncertain systematical relations recently. This is why the Discohelicidae, tentatively selected to accommodate Valamidiscus n. gen., is regarded here as a possible component in Subclass Euomphalomorpha Acknowledgements — Thanks for J. PÁLFY for calling my attention to these interesting finds. The studies on these gastropods are granted in frame of the T 031 873 project by the Hungarian National Research Found (OTKA).