foiind in the Nile hy M. Caillaud. 521 



the characters it presents prove its relation to the one or the other 

 of these two families, which are placed in two different orders. In 

 one of these orders, the Ostracea, the mantle is open and without 

 tubes or particular apertures ; in the other, the Cardiacea, which 

 comprehends the family of Chamae, the mantle, open in the front, 

 presents also two separate apertures for the excrements and respira- 

 tion. It is easily conceivable, that these characters, of far greater 

 importance than the number of muscular impressions, might prove 

 the propriety of placing the ^theria in the family of Ostracea, if the 

 animal presented the characters of the order to which it belongs. 

 Further, it is observable that muscular impressions present greater 

 variety than has generally been supposed. The division into one 

 or two great muscular masses is undoubtedly sufficiently precise, 

 but the less considerable muscles, whose impression is not strongly 

 marked, have been neglected. M. Cuvier in restoring the genera 

 Crenatula, Avicula, Pinna, &c., placed by Lamarck in the Mono- 

 myaria, to the Acephala dimyaria, has already shown that this 

 character may lead into error. Thus the order of Ostracea, to 

 which the ^Etheriae seem to be most nearly related, contains also 

 Acephala dimyaria, among which they ought to be counted. We 

 shall add that several other shells present three and even four dis- 

 tinct impressions, and that the number of muscles of attachment 

 does not appear to be confined to the two masses commonly dis- 

 tinguished ; (almost all the Uniones and Auodonti present four 

 separate muscular impressions;) and we have never yet distin- 

 guished in a comparative manner, which of the muscles of attach- 

 ment acquires a predominance over the others, according to the 

 form of the animal, the size or form of the valves, &c. 



However, the shells brought by Mr. Caillaud, not belongiui^ 

 to the genus Ostrea, it cannot be thence concluded that Oysters 

 formerly lived in fresh wiiter, but it would not be surprizing if 

 beds should be met with in which shells at (irst taken for Oysters 

 should prove to be iEtheriae. 'i'he discovery which proves that 

 this genus belongs to fresh Mater, is an interesting fact which 

 appears to deserve the attention of Naturalists. 



The genus vEtheria was so little known some years ago, that 

 M. Cuvier has not mentioned it in his Regno Animal, but it has 



