1 82 CHAM A. 



chalcedony. The principal hinge-tooth is oblique 

 and very strong, and the inside of the shell is lined 

 with a white, opaque layer, beautifully crenulated at 

 the edge. The general shape of the shell is circular, 

 but it varies greatly in form, according to its sur- 

 roundings. It is an inch or two in diameter. 



Its name is Chmna pelhccida^ 



Sby., Ka^ma pel-lu'-si-da. It 



is always found firmly 



attached to a rock by its 



lower valve. So strong is 



the adhesion that you must 



break off a piece of the rock 



or you will sacrifice a part of 



your shell. It is very easy 



to overlook them, as they 



Fig, 155 appear like ragged knobs on 



the rock, but when once you have collected a good 



specimen you will admire its peculiar beauty. 



The name Chama is very old, having been men- 

 tioned by Pliny; the specific name, pelhicida^ beau- 

 tifully refers to the pure and translucent appearance 

 of the shell. 



Chama exogyra^ Conr., ex-o-gy'-ra. This is a 

 southern species, similar in size and shape to the last, 

 but having a coarser, more oblique shell. The chief 

 difference, however, is seen in the curv^e of the um- 

 bones. If you stand a specimen of this species on 

 its edge, with the beaks uppermost and curving 

 towards you, the side which was attached to the rock 

 will be towards your left hand. But if you place a 

 specimen of pellucida in the same position, the rocky 

 side will be towards your right hand. 



