VENEEID^E. 



61 



Mactra (Linn.). Shell nearly equal-sided, trian- 

 gular; liinge composed of a V-shaped cardinal 

 tooth in one valve, locking into a margined pit 

 in the other, and of a long lateral tooth on each 

 side, which fits into a deep groove with tooth-like 

 edges. Animal with the mantle open to the 

 siphons, fringed at the edges ; foot heeled. 



M. elliptica. M. stultorum. Fig. 87 ; J n. s. 



helvacea. subtruncata. 



solida. truncata. 



FAMILY VIII. VENERID^E. 



Shell regular, closed, round or oblong ; ligament 

 external ; hinge usually three diverging teeth in 

 each valve ; muscular impressions oval, polished ; 

 pallial line sinuated. Animal free, creeping by 

 means of a tongue-shaped, flattened foot; mantle 

 generally with a large opening in front ; siphons 

 unequal, more or less united. 



Petricola (Lamk.). Shell oblong, thin, swollen ; 

 front side short. Animal with the mantle closed 

 in front ; foot small, pointed. Bores into lime- 

 stone. 



P. lithopliaga. Fig. 88 ; n. s. 



