Unio only, and, as Ferussac had already done, to consider 

 the other genera as subgenera. 



Blainville describes the animal nearly thus : Body large, 

 thick, more or less oval ; mantle thickened on the margin, 

 simple or fringed, and excepting on the back, open all 

 around ; anus oval, distinct ; a kind of small incomplete 

 tube, furnished with two ranges of cirri, for the respiratory 

 cavity ; foot very large, compressed, lamelliform. 



The principal naturalists and anatomists have been de- 

 cidedly of opinion that the animals of this family are her- 

 maphrodites ; but Mr. Prevost of Geneva affirms that he 

 observed, in some individuals of the Unio pictorum, the ex- 

 istence of spermatic animalculse, which he could not per- 

 ceive in those which contained eggs. He therefore infer- 

 red that the sexes were distinct. This led Blainville to a 

 reexamination of the subject : he dissected about forty in- 

 dividuals of the genera Unio and Anodonta, without discov- 

 ering any indications that could lead him to suppose the 

 existence of the male sex ; still however he is in doubt, and 

 we are very much inclined to believe, with Ferussac, that 

 Prevost may be right, but that more observations and ob- 

 servers are required fully to establish this disputed point, 

 although Baer has gone far towards even this object. — 

 Treviranus also made some interesting obseiTations on this 

 subject, an account of which he published in the Zeitsch. 

 fuT Physiol, in 1 824. He was of the opinion that the same 

 organ produced both the ova and the fecundating fluid. 

 He however remarks that he found, at the season of exclu- 

 ding their eggs, many that were entirely destitute of them. 



Some naturalists have changed the designation of this 

 genus to Anodon, as being more rigidly correct. 



PLATE XI. 



