﻿PLATYOSTOMA. 201 



Hence the aspect of the shell varies much with its age, or with the point of view 

 from which it is observed. Sometimes it appears almost lenticular, and at others 

 subglobose. Nevertheless it may be regarded as a very well-defined species, for it 

 has clear distinctive features in spite of this tendency to individual variation. Thus 

 the spire has a peculiar and characteristic style of coiling ; the body-whorl has 

 one or two spiral flattenings, which break the uniform convexity of its section ; the 

 rapid increase and change of shape of the tube gives a characteristic spiral slope 

 to the base ; the aperture has an incurved inner lip, and a break between the 

 upper terminations of the two lips ; and the style of ornamentation is altogether 

 very unusual and distinctive. This ornament consists of thin, parallel, micro- 

 scopic strise, arching backward from the suture until they become almost 

 transverse, and then changing their curvature, and becoming almost longitudinal 

 upon the lower part of the whorl. These lines appear to be structural rather 

 than mere surface-markings, but, as the shells are generally decayed, they can 

 only occasionally be seen. 



I am strongly of opinion that this species is the shell referred to by 

 Sowerby in the ' Encyclopaedia Metropolitana ' as N. spirata, and described in the 

 ' Geological Transactions ' as N. deformis. His description is very slight, and his 

 figure is so poor that neither Mr. Roberts nor myself could feel any certainty 

 about the correctness of its identification with the present shells, some of which 

 have been so labelled in the Godwin- Austen Collection. They certainly can only 

 belong to Sowerby's species if his figures in the ' Geological Transactions ' are 

 inaccurate. The back view might agree with them if a little tilted up, but the 

 front view appears to depict a nonumbilicated shell with continuous lips. Hence 

 they ought to represent a shell even of a different genus from the present ; but, 

 nevertheless, after a careful comparison of his figures and description with some 

 of the Godwin-Austen specimens I feel very much under the impression that the 

 appearances in his figure are only due to shading, and that the type specimen 

 from Plymouth will be found to belong to the same species as these Wolborough 

 shells. 



It is to be noticed that Sowerby described it as finely striated, though the strias 

 figured are much coarser than those in ours. While, however, several such 

 differences could be pointed out, there is in it such a look of specific resemblance 

 to our fossils that I think they must really be due only to the roughness of 

 Sowerby's drawing. 



Affinities. — Nerita spirata, Sowerby, 1 from the Mountain Limestone, is very 

 similar, but its ornamentation has a different contour, the shape of its aperture 

 is different, and it has a spiral concavity encircling the whorls just below the 

 suture, of which there is no trace in any of our specimens. 



1 1824, Sowerby, ' Mm. Conch.,' vol. v, p. 93, p], cccclxiii, figs. 1, 2. 



