84) Mr Gray on Testaceous Molliisca, 



of the mantle so essential a distinction as to found on it his di- 

 vision of the bivalves into families. In his system, therefore, 

 the genus Dreissena would be placed with the family of Cha- 

 macees^ while the genus Mytilus forms the type of the preced- 

 ing family of Mytilacees, The genus Iridina, however, and one 

 or two others, shew that this character cannot be implicitly re- 

 lied on for the natural classification of animals of this class, al- 

 though it forms a very good generic mark of distinction. 



The genus Iridina* above referred to affords a' second in- 

 stance of this anomaly ; for though the animals of the Iridints 

 and Anodontce differ in the adhesion and non-adhesion of the 

 lobes of the mantles, yet the shells are so alike that they can- 

 not be distinguished by any external character ; so much so, 

 that one of the species now referred to the genus by M. Des- 

 hayes, who first pointed out this peculiarity in the animal, was 

 considered as an Anodon by Lamarck. 



The animals of Cytherea, Venus^ and Venei'upis have, like 

 those of most of the alHed genera, a lanceolate foot projecting at 

 the anterior part of the shell ; while the genus Artemis of Poli, 

 which has generally been confounded with Cytherea, from which 

 it is not easily to be distinguished except by its usually more 

 rounded form, is provided with a crescent-shaped foot, exserted 

 at the middle of the lower edges of the valves. 



Again, there is but little difference in external characters and 

 habit between Cyclas and Pisidium ; but the animals of the 

 latter have elongated syphons, which are not found in the former. 



In reference to Univalves it may also be observed, that it is 

 frequently impossible to distinguish some of the genera of that 

 class without an examination of their opercula. This is the case, 

 for instance, as regards the smaller and more solid Paludina, 

 inhabitants of fresh water, and some species of Littorina living on 

 the coast; several of the shells described, as Paludina, by Drapar- 

 nauld and others, appearing rather to belong to the latter genus. 



* Lamarck formed this genus on a specimen which had its hinge margin 

 accidentally tubercular and slightly crenated ; but this character is not found 

 in most of the specimens of the species which he describes. The English 

 conchologists, misled by this character, have referred to the genus a very dif- 

 ferent African shell, with a long series of transverse teeth on the hinge mar- 

 gin, which has lately been separated by Mr Conrad under the name of Pleio- 

 don. 



