250 CRETACEOUS PELECYPODA 



The species of Lucina, tbougli not so numerous as those of TelUna, offer a 

 soinewhat similar amount of variation. Althoixgh some of them appear at first 

 sight to represent very different types, they are upon a comparison of Lirgc materials 

 connected by so many intermediate forms that it does not seem advisable to give 

 them a higher than a sub-generic rank. Deshayes in his valuable contribution 

 to the anatomy of the Lucime (Journ. de Conch., 18G1, ix, p. 317,) has, I think, 

 su.fficiently proved that there is an intimate connection existing between all the 

 recent species of Lucince, all possessing in common the more or less orbiculate form, 

 moderately curved hinge-line either with well developed cardinal and lateral, or 

 with partially or wholly obsolete teeth, a large anterior muscular impression, rxigose 

 surface of the internal shell, the mantle being partially attached to it, &c. The 

 animals in all the species which Deshayes examined were very similar. At the 

 same time, however, it cannot be denied that there are several well marked groups 

 of Lucina, and following H. and A. Adams and others, I shall retain them, as 

 already noticed, as sub-genera. They are, it is true, chiefly based upon some 

 variations in the external shape and upon the ornamentation, characters which 

 at first sight appear very subordinate. But there can be no doubt that several 

 of them are well founded distinctions, which can be traced from the present time 

 through all the tertiary, cretaceous, and in some cases even into the Jurassic deposits. 



a. The name Lucina has been reserved for the species of the type of 

 L. Jamaicensis, Spengler; the shells are roundish, oval, or orbicular, concentrically 

 lamellated or striated, moderately tumid ; cardinal and lateral teeth, as a rule, well 

 developed. 



6. Loripes, Poli, 1791, type, L. lactea, Linn. ; similar in form to Lucina, but 

 usually more tumid, either the lateral or the cardinal, or all the hinge-teeth, become 

 obsolete ; ligament sub-internal, sometimes quite closed up in a furrow of the poste- 

 rior hinge margin. 



(a). There are four somewhat different types of recent Loripes, the one repre- 

 sented by the above-mentioned type species, and a few others ; the shells are usually 

 moderately compressed, orbicular, with fine concentric strite on the surface ; the 

 rio'ht valve has one rather large cardinal tooth, the left two smaller teeth, the ante- 

 rior bein"" placed parallel to the lunular margin; the ligament is almost quite 

 internal. This group appears to be purely marine. 



(/3). The second group is represented by species like X?«(7. 5a?-ia/rt, o?;?;/;*, &c. ; 

 they are generally strongly inflated and have all the hinge-teeth obsolete or nearly 

 so ; these are also mostly purely marine shells. 



(y). The third sub-division is represented by Luc. FJiiVqipinarum, Hanley ; the 

 shells are comparatively thick, concentrically coarsely lamellated, covered with a 

 horny epidermis, the teeth are either obsolete, or there is one blunt cardinal 

 tooth in one and a simple groove in the other valve ; usually the anterior, but 

 very rarely the posterior lateral teeth are developed, all, as a rule, become obsolete 

 in old specimens. The species belonging to this group mostly live in brackish 

 water and in estuaries. 



