﻿342 



EOCENE MOLLUSCA. 



tion to which some of the species of this genus have been subject, and the consequent 

 uncertainty which attaches to their identification. 



Sandberger has figured and described a British fossil under the above name elongatus, 

 which, he says, was received from Mr. Edwards, but his figure is rather less elongated 

 than are my specimens. 



1 have not seen any specimen of Limnaa from the Lower Eocene of England, neither 

 have I seen any fossil from either our Upper or Lower Eocene beds that can be referred 

 to the genus Physa, though several species of that genus have been figured by 

 M. Deshayes from the Paris Basin. 



Fam. — NERITIDiE. 



Genus 32nd. — Neritina. Lamarck, 1809. 



Generic Character. " Testa tenuis, semiglobosa vel ovalis, subtus planulata, non 

 unibilicata ; aperturd semi-rotunda ; labro columellari planulato ; margine acutiusculo 

 subrecto, plerumque denticidato, labro externo intus nec dentato nec crenulato ; operculum 

 testaceum semicirculare ; interne appendice laterali instructum." 



This has been separated from Nerita, and intended for those species which inhabit fresh 

 water; but there is little or no difference in the form and general character of the shells of 

 the two genera, and most of our present species inhabit waters that are neither salt or 

 fresh, as it is to be presumed did their fossil congeners. They can only be distinguished 

 under the above respective generic names by the palaeontologist, according to the 

 indication of habit which at their association with either known marine or estuarine species 

 affords, although among living species there are two (N. viridis and N. meleagris) which, 

 belonging to the section grouped as Neritina, nevertheless, are found in the sea. 



This diagnosis of Neritina, as given by Lamarck (tenuis), is not restricted to the 

 number of shells that have have been figured and described under this generic name, 

 several of them being as thick and ponderous as many of the species called Nerita. 

 Some are not externally smooth, but are ornamented with ridges and carinas ; others have 

 denticulations more or less upon the outer and inner lips, while the opercula of Nerita 

 and Neritina are thick and possess the same characters alike. M. Deshayes, in the 

 second edition of Lamarck's ' Hist. Nat. An. sans Vert.,' vol. viii, p. 565, has made some 

 very just remarks upon the similarity of these genera ; and in his last work, ' Hist, des 

 An. sans Vert, du Bas. de Par.,' he has grouped all these hitherto called Neritina as 

 simply a section of Nerita. As, however, I am describing land and fluviatile shells of 

 the older Tertiaries, I have thought it best to retain the name of Neritina for those 

 species which, if not restricted to a habitat in fresh water, are met with where the water 



