COLLECTION OF SHELLS. 5l9 



cies known ; it was brought from New Holland 

 by Per on. Many others are found in the same 

 strata with the ammonites. 



The family of the nayades is altogether com- 

 posed of river shells. Of the four genera into 

 which it is divided, the most numerous is the 

 unio, of which there are forty-six species in the 

 collection, nearly all inhabiting the lakes and 

 rivers of North America. We are indebted for 

 them to MM. Michaux, Lesueur and Milbert. 

 The unio pictorum and the littoralis are found in 

 the Seine. The two species of the genus hyria 

 come from Ceylon. Amongst the nine species 

 of the genus anodonta the a. cignorum lives in 

 our ponds, and is so named because it is eaten 

 by swans. 



The eiherice, the chamce, and the dicerates com- 

 pose thie family of the chamacece. We have but 

 one species of etheria, the elliptica, from the Per- 

 sian gulph, which is the rarest, largest, and most 

 beautiful of the genus 



The second and last order of the conchifera is 

 that of the monomyaria, which is divided into 

 seven families. 



i st. That of the tridacn?ce, formed of the two 

 genera tridacna and hippopus. We have six spe- 

 cies of the tridacna, one of which, the chama 

 gigas, Lin., is sometimes more than 6 feet in cir- 



