66 EOCENE MOLLUSCA. 



(Helicetta, Lamarck,) is somewhat discoidal, with a slightly elevated spire formed of 

 about four whorls, generally rounded or bluntly convex, but which, in two casts of 

 fully grown individuals in my cabinet, present a sub-carinated periphery. The 

 surface is covered with numerous regular raised lines, separated by shallow rounded 

 sulci; the lines are oblique, undulating, and rounded. The margins of the 

 depressed semilunar aperture are simple and unreflected. The umbilicus is moderately 

 wide. 



Mr. S. Wood, in his ' List of Shells from Hordwell Freshwater Bed/ has referred 

 this shell to the North American species, H. striatetta, Anthony; but, although I feel 

 great hesitation in dissenting from his opinion, the differences between the two render 

 it difficult to maintain their identity, at all events, before we are better acquainted with 

 the influence of external conditions in modifying the forms of the animal and its shell. 

 I should add that I have only one specimen with the shell preserved, (the one referred 

 to by Mr. Wood, and which he has been kind enough to add to my collection,) and 

 that this specimen is in an imperfect state. On comparing this shell with the recent 

 H. striatetta, it will be seen that in the latter species the spire is more elevated, the 

 lineation sharper, the sulci not so deep, the whorls wider, rounder, and less embracing ; 

 the suture not so depressed, and the aperture larger. Similar differences exist between 

 this and H. ruderata, a species from Cincinnati described by Binney. In H. perspectiva, 

 Say, which it somewhat resembles, the spire is more depressed, the lineation, like that 

 of H. striatetta, is fainter and sharper, the volutions more numerous, the peritreme 

 more distinctly carinated, and the umbilicus wider.* 



A shell occurs in the Pleistocene freshwater deposit at Clacton, which is referred 

 to H. ruderata : the striation resembles that of the present shell ; but in other respects 

 it very closely resembles the American shell. M. Deshayes has described a fossil 

 shell from the upper freshwater formation of the Soissonnais (//. Ferrantii), to which 

 this species presents a general resemblance ; but it is separated from that shell by the 

 more elevated spire, and the more numerous whorls ; and in H. Ferrantii the raised 

 lines appear to be fewer and less regular, and the umbilicus to be narrower. 



Size. — Diameter | of an inch, nearly ; elevation 1-1 Oth inch. 



Localities. — Hordwell Cliff; Sconce. 



* The H. striatella of Anthony is from Massachusetts, and, until recently, was considered to be merely 

 a variety of Say's H. perspectiva, which is from Ohio and Lake Erie. Gould, in his ' Report on the 

 invertebrate Animals of Massachusetts,' has pointed out the distinctions. The H. ruderata of Binney is 

 from Cincinnati, and has also been considered as a variety of H. perspectiva ; it appears to belong rather to 

 H. striatella. 





