GENERA OF SHELLS. 13 



GLYCIMERIS. Shell transverse, widely gap- 

 ing at each side ; hinge callous, toothless. 

 Nymphse prominent ; ligament exterior. 



The glycimeris differs from solen and saxicava, by 

 having the ligament on the shortest side, and also from 

 solen, by having no cardinal tooth. 

 G. siliqua. arctica. 



MYARIA. 



Ligament internal ; hazing one large spoon-shaped tooth in 

 each valve, or in one only, to the cavity of which the liga- 

 ment is attached. Shell gaping at one or both sides. 



MYA. Shell bivalve, transverse, gaping at 

 both ends. Left valve with a large, com- 

 pressed, rounded cardinal tooth, projecting 

 almost vertically ; a cardinal fossa in the other 

 valve ; ligament interior, inserted into the tooth 

 and fossa of the opposite valves. 



M. truncata, arenaria, erodona, solemyalis, acuta, mer- 

 cenaria, hyalina. 



ANATINA. Shell transverse, subequivalve, 

 gaping at one or both sides, cardinal tooth 

 naked, dilated, spoon-shaped, projecting in- 

 wardly, inserted on each valve, and receiving 

 the ligament ; a lamina or falciform rib adnate 

 and running obliquely below the cardinal teeth 

 in most. 



Anatina differs from my a, by having two spoon- 

 shaped teeth, instead of one. 



A. laterna, truncata, subrostrata, longirostris, globulosa, 

 trapezoides, rugosa, imperfecta, myalis, rupicola papy- 

 ratia, Leana. 



