LAMELLIBRANCHIA. 
39 
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, 
A. TJnio graniferus. 
B. Anodonta anatina. 
Fig. 31, B), whilst in others it is enormously thickened and furnished 
with strong interlocking teeth (see Fig. 31, A). 
The family JEtheriidm, or freshwater Oysters, consists of but Case 178. 
three genera : JStheria contains African, and Mulleria and Bartlettia 
Indian and South-American forms. When young the shells of 
JEtMria (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 
