AIVIPHIDESMA. 



GENERIC CHARACTER. 



Shell rounded or transversely somewhat oval, subinequi- 

 lateral ; hinge witli one or two cardinal teeth, and having a 

 lateral tooth each side, sometimes obsolete ; a slender 

 oblique fosset originating immediately under the beak and 

 passing anteriorly : ligament divided into two parts, of 

 which the true ligament is exterior and very short, and the 

 cartilage portion is interior, affixed in the oblique fosset : 

 muscular impressions two, remote, not elongated : palleal 

 line with a much dilated and profound sinus. 



OBSERVATIONS. 



The characters of this genus are very distinct and easily 

 recognized from all those that have no part of the liga- 

 ment internal. Lamarck first distinguished it by the name 

 of Donacilla,which I think ought to bc[retainedon the prin- 

 ciple of priority, and that of Amphidesma, that he after- 

 wards applied to it, ought to be rejected. He has associa- 

 ted it. in tlic same small group, with Ungulina and Sole- 

 mya, from both of which it may be known by its much 

 dilated and profound sinus of the palleal line. Other gene- 

 ra of his family of Mactraceai have divided ligaments, par- 

 ticularly Mactra, with which, in fact, Linne arranged the 

 species ; but their cardinal fosset is more direct, dilated and 

 deltoid, whilst that of Amphidesma is nearly parallel with 

 the edge and fusiform. Lutraria is entirely destitute 



