CYRENA. 



Lam.^JIist. Nat. des Anim, sans vert. t. 5. p. 551. 



■ '-i> 



TESTA rotundato-subtrigona, ventricosa; aequi- 

 valvis, in^quilatera, dentibus utriusque valvas 

 cardinalibus tribus, lateralibus duobus, postico ad 

 cardinem propius admoto, antico remoto, Im- 

 pressiones musculares duae, laterales, distantes, 

 Ligamentum externum, lateri majori insertum. 



In forming his Genus Cyrena, Lamarck seems to have 

 depended for his distinguishing characters, principally 

 upon the shells which constitute it being possessed of 

 three hinge teeth ; upon their umbones being much eroded, 

 and upon their being in general thick and large shells : 

 much experience is not, however, necessary, to prove how 

 little dependance must be placed upon such characters as 

 those of size and erosion of the umbones. As distinctive 

 characters between one Genus and another, or one species 

 and another, these can be of very little importance, parti- 

 cularly with respect to fluviatile shells, because we see 

 that all such shells are commonly eroded, many of them 

 are very large, and the thinner are constantly less eroded 

 than the thicker. The situation, number and peculiarities 

 of the teeth and hinge must, therefore, still be recurred 

 to as the best generic distinctions in several families of 

 bivalve shells. It appears to us convenient to follow 

 Lamarck in separating the Genus Cyrena, though it is not 

 distinguished by Bruguiere from Cyclas; its species are 

 placed by Gmelin among the Veneres andTellinas; they 

 are all fluviatile shells, and covered with a strong epi- 

 dermis; we believe none of them are found in Europe, 

 but that they are all inhabitants of warm climates. Of 

 recent species there are not very many : Lamarck mentions 

 eleven ; two or three that are not described by him are 

 known to us, one of which we have represented. La- 

 marck's two divisions of the Genus are founded upon the 

 presence or absence of striae upon the lateral teeth ; ours 

 belongs to the second division, which is destitute of these 

 striae. 



