304 MR. GRAY ON TESTACEOUS MOLLUSCA. 



the animal of Mytilus polymorphus they are united through nearly their whole extent, 

 leaving only three small apertures, one for the passage of the foot and beard, and the 

 other two for the reception and rejection of the water, from the contents of which the 

 animal derives its sustenance. This shell must consequently form a new genus, to 

 which the name of Dreissena has been appropriated by Van Beneden *. As a proof 

 of the importance attached to this character, it may be observed that Cuvier con- 

 sidered the adherence or non-adherence of the lobes of the mantle so essential a 

 distinction as to found on it his division of the bivalves into families. In his system, 

 therefore, the genus Dreissena would be placed with the family of Chamacdes, while 

 the genus Mytilus forms the type of the preceding family of Mytilacdes. The genus 

 Iridina, however, and one or two others, show that this character cannot be implicitly 

 relied on for the natural classification of animals of this class, although it forms a 

 very good generic mark of distinction. 



The genus Iridina^ above referred to affords a second instance of this anomaly; 

 for though the animals of the Iridina and Anodontcp. differ in the adhesion and non- 

 adhesion of the lobes of the mantles, yet the shells are so alike that they cannot be 

 distinguished by any external character ; so much so, that one of the species now 

 referred to the genus by M. Deshayes, who first pointed out this peculiarity in the 

 animal, was considered as an Anodon by Lamarck. 



The animals of Cytherea, Venus, and Venerupis have, like those of most of the allied 

 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 Cy- 

 clas and Pisidium ; but the animals of the latter have elongated siphons 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 oper- 

 cula. This is the case, for instance, as regards the smaller and more solid Paludince, 

 inhabitants of fresh water, and some species of Liftorina living on the coast ; several 

 of the shells described as Paludince by Draparnauld and others appearing rather to 

 .belong to the latter genus. A similar difficulty exists with respect to other Littorince 

 as distinguished from Phasianella, and with the Neritince as distinguished from the 

 Neritce. In the latter case the characters derived from the operculum are so essential 



* Institut, 1835, p. 130; and Annales des Sciences Naturelles, N. S., tom.iii. p. 193. 



t 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 different African shell, with a 

 long series of transverse teeth on the hinge margin, which has lately been separated by Mr. Conrad under the 

 name of Pleiodon. 



