520 M. de Ferussac on the MtherioR 



streams of this great arm of the Nile produce them also. The 

 number found upon the tombs throughout Ethiopia is so great, 

 that it is astonishing that Bruce and Burckhardt should not have 

 mentioned them. 



We had already begun to suspect that these interesting Shells 

 might not be Oysters, and at first sight we discovered that they 

 were JEiheriee^ an extremely rare genus, kuowu only in a few 

 Cabinets, and instituted by Lamarck in the Annales du Museum. 

 This celebrated author has described the ^therice as in- 

 habitants of the depths of the sea, although he had no further 

 proof in support of this opinion, than their relation to the 

 Ostreae, and in fact, it is difficult to distinguish them at first 

 sight. The species brought by M. Caillaud bears so striking a 

 resemblance to some Oysters, that he may be well excused for 

 not having discovered the differences, the principal of which 

 consists in the iEtheriai having two muscular impressions, while 

 the Ostreae have only one. This difierence has induced M.La- 

 marck to place the ^theriae in the family of the Chamae, con- 

 sequently very far from the Oysters, to which they possess such 

 great analogy, both in the general appearance (fades) of their 

 valves, the manner of life of their animals, and particularly by 

 the displacement of the upper valve, and the elongation of the 

 beak of the lower valve, characters peculiar to the Ostreae and 

 Spondyli. The question might here be raised, whether the con- 

 sideration of the number of muscular impressions, should, in a 

 Classification, founded upon natural bases, be regarded as of 

 more importance than such marked and near analogies. Accord- 

 ing to M. Blainville, it is probably wrong to separate the^ j^theriae 

 from the Ostreae, for " it is evident," says he, " that the number 

 of muscular impressions is not sufficient to dispose in a natural 

 manner, the calcareous envelopes of mollusca of this class." It is 

 nevertheless remarkable that the Chamae also possess important 

 analogies with the TEtheriae ; for like these latter and the Oysters, 

 they adhere to other substances and are deprived of locomotion, 

 they are irregular, inequivalve, &c., but their aspect is very differ- 

 ent, as well as the nature of their shells. It is tlierefore necessary to 

 examine the animal of the il^theria, in order to ascertain whether 



