CYTHEREA. 



valve, and the lateral tooth is generally very small^ and 

 closely approximated. 



One other important group remains to be distinguished 3 

 which leads directly to the t^^pical group of Lamarckian 

 Veneres, and apparently differing from them only in hav- 

 ing a distinct blunt lateral tooth, and in having no sinus 

 to the muscular impression of the mantle. Those La- 

 marckian Cythereae which have the muscular impression 

 of the mantle thus entire, are mostly smooth on the out- 

 side ; some of them, however, are concentrically striated, 

 and others have diverging striae and ribs ; their umbones 

 are mostly inclined forwards. In these the lunule is more 

 distinctly marked than in most others. C. pectinata, gib- 

 bia, castrensis, scripta, omata, and other well-known spe- 

 cies belong to this group. 



Other groups, distinguished by less prominent charac- 

 ters, and of smaller extent, occur; one of these it may 

 be proper to particularize ; it consists of only three or 

 four species^ which have been retained among the Donaces 

 by Lamarck, namely, C. meroe and its cognate species 3 

 these are remarkable for their nearly regular oblong form, 

 their central umbones, and a deep sulcus formed between 

 the valves immediately behind the umbones : they have 

 scarcely any circumstance in common with the Donaces. 



We have drawn up the following character of this Genus 

 in such a manner as to admit all the groups mentioned 

 above ; much greater precision would of course be neces- 

 sary, were it thought prudent to establish so many different 

 genera. 



Shell bivalve, equivalve, generally more or less inequi- 

 lateral, obtusely triangular, ovate or lenticular, smooth 

 or variously striated ; with three or more diverging hinge 

 teeth, and one anterior lateral tooth in each valve j the 

 lateral tooth more or less approximated to the hinge teeth. 



Cytherea is nearly related to Venus and Cyprina^ differing 

 however from both in having a lateral tooth ; it appears to 

 pass into Venus by those species which are nearly lenticu- 

 lar in form, and whose lateral tooth is brought very close 

 to the hinge teeth, for we find in some of Lamarck's typi- 

 cal Veneres a small vestige of an approximated lateral 

 tooth. The form of the muscular impressions will distin- 

 guish the Cythereae from the Lucince, 



The fossil species of this extensive Genus belong princi- 



