50 



althoUjih external, has, like our fluviatile shell, a channel (not mentioned 

 by Lamarck) communicating with the interior of the shell : and it 

 appears deserving of forming a separate genus intermediate between 

 Solen Ensis and its affinities and the genus under consideration. 



Our shel lis easily distinguished from the Solens, which most nearly 

 approach to it by its prominent beaks, its irregular form, and ihe great 

 length of its siphonal scar. At times some of the teeth become obso- 

 lete, as in Solen, and both the cardinal and basal edges are subject to 

 slight emargination. 



Long before meeting with the live animal, I had predicted the extra- 

 ordinary length of the siphons from the appearance of the siphonal scar 

 which, as Mr. Gray has well observed in the Zoological Journal, is a 

 good auxiliary character for the classification of Bivalves. 



Except Mr. Gray's new Chinese genus Glauconome, no other fresh- 

 water shell has a long siphonal scar. The remaining Conchae Fluvia- 

 tiles, and the whole of fhe Naiadae, having but a slight emargination in 

 the submarginal impression, and their ciliated siphons scarcely pro- 

 jecting beyond the extremities of the shells. 



This shell is chiefly interesting as being the first of the family of Sole- 

 naceae or even of the Crassipeda, which has been ascertained to inhabit 

 fresh water, and must be peculiarly so to the Geologist, who can ill pro- 

 nounce upon the nature of the medium which was inhabited by a fossil 

 shell under investigation, until all the genera which inhabit fresh water 

 are known. I must confess, however, that it has appeared to me, that in 

 geology too much stress has been laid upon shells, and that the water 

 which deposited them has often been hastily assumed as fresh, from the 

 examination of the exuviae found in a particular stratum, to which cur- 

 rents and other extraneous causes might easily have conveyed them from 

 some vast antediluvian River. 



Mr. G. B. Sowerby, to whom I forwarded dead, and therefore imper- 

 fect specimens of the Shell in 1828 will have hardly failed to characterize 

 the genus, as far as the specimens and remarks furnished would admit 

 of, should he have received them safely ; but as that may not have been 



