CROSSOSTOMA. 345 



Family— TURBINIDM. 



" Shell spiral, turbinated or pyramidal, nacreous inside; operculum calcareous 

 and pauci-spiral, or homy and mnltisipiral" — S. P. Woodward. 



The above diagnosis includes the Turbinidse, Trochidge, and Delphinulidge of 

 other authors, and, as far as Jurassic fossils are concerned, where the nacreous 

 layer is generally destroyed and opercula hardly ever to be found, cannot be 

 followed very closely. 



The withdrawal of Amberleya and the so-called " Littorina " reduces the number 

 of shells remaining to be considered under this family. These are for the most 

 part small. Whatever generic names are adopted, we recognise two groups, viz. a 

 group of smooth and a group of richly sculptured shells. 



Group I. — Smooth Shells. 



The smooth and generally small Turbinidae of the Jurassic Rocks have been 

 variously classified. The following are amongst the genera which have been 

 invoked : Ataphrus, Grossostoma, Ghrysostoma, Monodonta, Pleuratella, Plocostylus, 

 Rotella, Teinostoma, Trochopsis, Trochus, and Turbo. 



In the Inferior Oolite of this country I think that the smooth Turbinidas may 

 be focussed, provisionally at least, under three genera. Apart from one or two 

 forms which it may be convenient to arrange under Turbo (Section 1), the rest of 

 the shells exhibit a sequence from the most depressed form — Grossostoma Prattii, 

 to the most acutely trochiform Monodontoid, viz. Ataphrus Acis. Mr. Tawney, it 

 may be remembered, only recognised two species in his ' Dundry Gasteropoda,' 

 viz. " Monodonta " laevigata, Sow., and " Monodonta " Acmon, d'Orb. 



Genus — Crossostoma, Morris and, Lycett, 1851. 



" Shell thick, turbinated, smooth, somewhat depressed or rotelliform ; whorls 

 more or less flattened, few ; apex of the spire obtuse, depressed ; aperture nearly 

 circular, entire; the columella forms at its base a simple prominent obtuse tooth ; the 

 outer lip is smooth ; there is no umbilicus " — Morris and Lycett. 



Lycett, in notes and corrections to the ' Supplement,' p. 153, withdrew 



