PELECYPODA. 



39 



and the shells of the females are somewhat more tumid than those 

 of the males. Margaritana margaritifera (Case 168), which is found 

 in this country and in Europe, sometimes produces handsome pearls, 

 but not equal to those obtained from the pearl-oyster of tropical 

 seas. The hinge in this family is extremely variable, being in some 

 instances delicate and toothless {Anodonta, Fig. 31, B), whilst in 

 others it is enormously thickened and furnished with strong inter- 

 locking teeth (see Fig. 31, A). 



Fio-. 31. 



Anodonta anatina. 



The family ^theriidce, or freshwater Oysters, consists of but 

 three genera : ^theria contains African, and Mulleria and Bartlettia 

 Indian and South-Americau forms. When young the shells of 

 jEtlieria (which are common in the Nile) are free and not unlike 

 an Anodonta, but when adult they become attached and irregular 

 and look like an olive-green Oyster ; they are, however, provided 

 with two muscular impressions instead of one, as in ordinary marine 



Cases 178- 

 179. 



