352 INTKODUCTION TO CONCHOLOGY. 



the Solenacea and Mactracea, partaking of the characters of 

 both. Their shell is generally gaping on both sides, with 

 an internal hgament inserted in the hollow of a prominent 

 spoon-shaped tooth in one or both valves. Occasionally, 

 however, the ligament is external, and it is then partially 

 covered with a small testaceous appendage. Such are the 

 characteristics of the My aria, to which eight genera per- 

 tain — Mya, Anatina, Thracia, Corhula, Pandora, Anatinella, 

 Myocliama, and Cleidothcerus. 



The genus Mya, which serves to coimect this family with 

 the Solenacea, is peculiarly characterized. The shell is 

 transverse, inequilateral, gaping at both ends, and covered 

 with a thick epidermis, which is continued at the end over 

 two lobes, protruded by the animal in making its way into 

 the sand. A large prominent spoon-shaped tooth is con- 

 spicuous in one valve, with a corresponding pit or cavity in 

 the other, between which the ligament or cartilage is in- 

 serted. This tooth displays a very curious variety of cell- 

 structure, in which a layer of large cells, occupied by car- 

 bonate of lime, disposed in a radiated form of crystallization, 

 resembles that of the mineral called WaveUite. Traces of 

 this beautiful arrangement are also discoverable in other 

 instances. 



