T56 G. LINDSTRpM, ON THE SILURIAN GASTROPODA AND PTEKOPODA OF GOTLAND. 



Gen. ORIOSTOMA Munier-Chalmas. 



1814 Euomphalus Sow. p. p. Miu. Conob. vol. 1, 97. 



1864 Omphalotroclius Meek. Rep:t. Geol. Survey of California, PalKont. vol. I, 16. 



1876 Oriostoma Mun.-Ch.\i,m.\s Journal tie Conchyliologie vol. XVI, 103. 



1881 Potytropis De Koninck Faune Carbonif. II,iii 107. 



Shell discoid, with short spire, rarely high, whorls tubular, iientricose, joined, seldom 

 a little disjointed near the aperture, longitudinally sculptured with prominent heels, aper- 

 ture li'ith thin lips without any sinus, generally continuous, umbilicus wide and open. In 

 several species the nacreous lustre of the interior shell strata is retained on the nucleus. 

 The operculum is calcareous and solid, on the inner side smooth with a thick, elevated 

 rim around the margins, outside conical, sometimes higher than broad, covered by a number 

 of spiral coils ornamented with exceedingly thin lines. Mostly large shells, but some species 

 also small. 



Besides the above, in the list of synonyms enumerated genera, which liiive ex- 

 clusively or almost so been applied to the species of tliis genus, there is also a num- 

 ber of other genera, in wiiich some of them have been included as in Turbo by Pic- 

 TET, Delphinula by Htsinger and others, Trochilites and Helicites by Schlotheim, Tur- 

 binites by Wahlenberg and Schlotheim. Straparolus Montfort might, according to 

 several authors, embrace species of this genus, but the first species described are not 

 known to have had any operculum and their ornamentation differs. If we strictly con- 

 fine ourselves to the species, which Soweeby, when he at first established his genus 

 Euomphalus, included into it, we must find that later created species such as E. 

 discors, E. rugosus, E. funatus etc. essentially differ from the former in having an en- 

 tire aperture without any slit as in those, which form the genus Euomphalus seiisu 

 strictiori and must be ranged near the Pleurotomaridas. Delphinula, employed by HisiN- 

 ger, is now applied to species from later formations and generally regarded as a sy- 

 nonym for Omalaxis, which has a chitinous operculum. Omphalotrochus Meek may in 

 some degree be related to or even contain species of this group, but the name indi- 

 cates an affinity, which is not borne out by the evidence given by the operculum of the 

 Silurian specimens. Polytropis De Kon. seems, to judge of the expressions of this author, 

 at first to have been intended instead of Inachus (In. sulcatus His. is a Pleurotomaria), but 

 De Koninck then cites Euomph. discors as the typical species and consequently the name 

 is synonymous with Oriostoma. The Devonian Oriostomata lately described by Oehlert, 

 as far as I am able to judge by specimens kindly sent from him, belong to the same 

 generic group as the Silurian, which formerly had been confounded with the Euom- 

 phali and both consequently must be placed in the same genus Oriostoma. The De- 

 vonian species however are all small and no operculum has as yet been found with 

 them. 



The distinction betAveen Oriostoma and Cyclonema, the next genus, is not quite 

 as clear as desirable, when tlie operculum is wanting. When an Oriostoma varies in 

 having a long spire, it nearly resembles a Cyclonema, but these never have the umbi- 

 licus as large as the former. 



