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, norany interlocking or matching together of the 
opposite folds. Tn 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 auriclesf 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 Naiades, 
and should be placed next to the Unic. 
W ivi enus 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. 
Seatice Dipsas of Dr. Leach, has the lamellar lateral tooth, but no cardinal 
} Processes on eash side of the beaks, like the seallop, Pecten opereularu. 
