﻿PLATYOSTOMA— CAPULUS. 203 



side, differing perhaps in the spire forming a slightly convex cone, and in the 

 mouth being smaller ; it is, however, at once distinguished by having (as described 

 by Goldfuss) a wide subangulated umbilicus. 



Platyostoma turbinatum, Hall, 1 approaches the present form so nearly as to 

 suggest that it very probably belongs to the same genus. The American species 

 is a decidedly more elongated form, with much fewer and more quickly increasing 

 whorls, and curved growth-lines of which we can trace no sign in our English 

 specimens. 



V. Family. — Capultdji, Guvier. 

 1. Genus. — Oapulus, Montfort, 1810. 



The palasozoic forms of this group were placed by Conrad under the genus 

 Platyceras in 1840, and by Phillips under the genus Acroculia in 1841 ; and these 

 names, as well as some others, have been largely used for them by various authors. 

 There is a vast amount of individual as well as of specific variation in the pateozoic 

 forms of these shells, and it appears to me that they include the recent species of 

 Gapulus within their limits, although on the other hand they contain many shapes 

 that would not otherwise be included in it. Thus, for instance, the spire is often 

 very asymmetrical and the surface tuberculated or strongly plaited. However, 

 these forms are so closely connected among themselves that it seems impossible to 

 draw any generic distinction between them, and therefore there seems no reason 

 for separating them from the recent genus, which contains only about ten living 

 species — a very small number compared with those of the Silurian and Devonian 

 formations, which are to be counted by hundreds. We therefore can only regard 

 the recent shells as the worn-out representatives of a large and flourishing 

 ancient type. 



At the same time, the horseshoe-shaped muscular impression which is so pro- 

 minent in the modern species is not generally noticeable in the Devonshire fossils 

 when they occur as casts ; and, if the shape of this proves to be different, it may be 

 needful to retain the name Platyceras as a sub-genus of Gapulus, or as a distinct 

 genus. At present, however, there seems no reason for doing this, and the best 

 course is to follow Tawney, Kayser, and others, in referring these ancient shells to 

 the genus Capulus itself. 



It has proved very difficult to decide the specific limits of such shells of this 

 genus as are described below. Hardly two specimens are exactly alike, and the 



1 1879, Hall, ' Pal. N. T.,' vol. v, pt. 2, p. 27, pi. ix, figs. 12—24. 



27 



