276 Mr. Barnes on the Genera Unio and Alasmodonta. 



OBSERVATIONS. 



This genus was established by Mr. Say. The shells are 

 distinguished from those of the genus Unio, by the want of 

 the lateral lamellar tooth, the place of which is commonly 

 occupied by a slightly elevated fold ; but no channel can be 

 perceived, nor any interlocking or matching together of the 

 opposite ;folds. In many specimens the part is perfectly 

 smooth. The sinus at the anterior termination of the lig- 

 ament is visible in all, and exactly resembles that of the 

 Unio ; as do also the colours of the epidermis and of the 

 interior ; but the polish of the inside is generally much less 

 brilliant than that of the Unio. The habit of the shells is 

 similar, many of them becoming thick and large. 



M. Lamarck seems not to have noticed this genus. He 

 makes the same remark of his Hyria,* that Mr. Say does of 

 this, " that together with the Dipsast of Dr. Leach it com- 

 pletes the connexion between the Unio and Anodonta.''^ 

 But no part of his generick character agrees with the pres- 

 ent genus except that the shell is equivalve, which is the 

 case in the whole family. 



The auriclesj of his Hyria and the lamellar tooth forbid 

 the supposition, that he could have intended this shell by 

 his description. It is however properly one of his Kaiades^ 

 and should be placed next to the Unio. 



We subdivide the Genus into two sections, commencing 

 with those that resemble the Unio and ending with those 

 that approach the Anodonta. 



Sections. 



* Shells thick and large. 

 * * Shells thin and small. 



* Generick character of the Hyria of M. Lamarck. Shell equivalve, ob- 

 Hquely triangular, auriculate, truncated and straight at the base ; hinge 

 with two teeth slightly elevated ; (rampantes.) the cardinal or posterior 

 tooth multipartite, divergent, the interior parts less ; the other or lateral 

 tooth very long and lamellar ; ligament external and linear. (Naker very 

 brilliant,) 



t The Dipsas of Dr. Leach, has the lamellar lateral tooth, but no cardinal 

 teeth. 



% Preeesses ou each side of the beaks, like the scallop, Pecien opercularis. 



