CO^Cil\ LIX— DITHYRA. 23. «7 



23. MACTRA — /./iViV^;. 



Testa subtrigona, cEquicalcis, laieiihtis subhianfibus. Cardo 

 dente cochleariformi, cum dente trigono adject o : lateralibiis 

 laminatis, valves alter his didymis. Ligament iim internum. 



Shell somewhat triangular, equivalve, a little open at the sides. 

 Hinge with a spoon-shaped tooth and an adjacent triangular 

 one: the lateral teeth laminar, double in one of the valves. 

 Ligament internal. 



Tlie shells confined to the above generic character form a very 

 natural family, remarkable for the number strength and position 

 of the teeth. In one valve there is a single elongated lateral plate 

 on each side, which closes between two in the other. And in this 

 respect they resemble the fresh-water shells of the genusMysca,but 

 the primary teeth are very different, and the ligament is internal. 



Those of the first division, or such as bave the lateral teeth 

 striate in a direction opposite to their transverse extension, are 

 usually thick and solid : and such as have the lateral teeth quite 

 plain, are thin and semitransparent. 



They inhabit the sandy parts of our coasts. 



^~ Denies laterales striati. 1-5. 

 The lateral teeth striate. 



Mactra testd ovato-trigond Ifeviusculd <equilaierali, lateribus ,0];^^ 



obliguis, timbonibus incurvis. 

 Shell oval-triangular nearly smooth and equilateral, with the 



sides oblique, and the beaks incurved. 



K 2 



I 



