126 CLASS I. TROPIOPODA. ORDER I. BIMUSCULOSA. 



consists of a single solid tooth, but it is sometimes obsolete. Two genera 

 only are referred to this family ; one is found to inhabit rivers, the other 

 is peculiar to the sea. 



Fluviatile . . Etheria. 



Marine .... Chama. 



ETHERIA, Lamarck. 



Testa irregularis, imequivalvis, adherens, periostraca crassa induta ; in- 

 terne glauco-viridis, submargaritacea, in vesiculas interdum inflata ; 

 umbonibus brevissimis, basi testa; subimmersis. Cardo edentulus, 

 undatus, subsinuosus, ina?qualis. Impressiones musculares oblongpe, 

 unica interdum fere obliterate! Ligamentum externum, contortum, 

 intus partim penetrans. 



The genus Etheria represents a small group of mollusks inhabiting the 

 great rivers of Central Africa, having a shell somewhat like that of the 

 Ostrea ; they differ, however, not only in being fluviatile, but in having 

 two internal muscles of attachment. They were but little known to 

 Lamarck ; he pronounced them to be marine, and ventures to assert that 

 they had escaped the notice of travellers on account of their living at- 

 tached to rocks at a considerable depth under water. When the Etheria? 

 were described by Sowerby in his ' Genera of Shells,' the animal was still 

 unknown, but from certain appearances on the shell he strongly suspected 

 it to be an inhabitant of fresh water. His suspicions were laudably 

 founded : in the first place, upon the shell being much eroded, like 

 that of the Naiades ; and, secondly, upon the outer surface being often 

 covered with the remains of those ovate, vesicular bodies so frequently 

 seen upon Neritina, &c, supposed to be the eggs of other freshwater 

 mollusks. This conjectural opinion has been singularly confirmed by 

 later discoveries ; the Etheria? have been found by Rang in the rivers of 



