406 DISSIMILAR ANIMALS IN 



Iridinae and Anodontae differ in the adhesion and non-adhe- 

 sion 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 Cyclas 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 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 Paludinao by Drapernauld and others appearing 

 rather to belong to the latter genus. A similar difficulty 

 exists with respect to other Littorina? as distinguished from 

 Phasianella, and with the Neritinae as distinguished from the 

 Neritae. In the latter case the characters derived from the 

 operculum are so essential to the discrimination of the two 

 genera, that M. Rang, looking only to the characters of the 

 shell, has proposed to reunite them into one. In proof of 

 the little attention that has hitherto been paid to this very 

 important part, I may mention that three species referred 

 by Lamarck to the genus Solarium are each furnished with a 

 different kind of operculum ; and it is deserving of notice 

 that the Monodonta canaliculata, according to the observa- 

 tions of M. Quoy, has an operculum very different from the 

 rest of the shells of that genus. 



" In some shells, again, the differences in character are so 

 slight as ahnost to throw an air of ridicule on the attempt to 

 separate them generically from the structure of the shells 



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. 



