126 LEA ON THE GENUS ACOSTiEA OF D'ORBIGNY. 



The ligament continues on the side of the valves. The left valve now becomes the 

 superior one, and commences to open itself on the anal region. It then continues 

 slightly open for some time, after which this valve separates itself entirely, from all 

 that characterised it in its early age, to become irregular, like the opposite valve, 

 detaching itself entirely from its early age, (the two valves of its early existence) 

 which remains attached to the inferior valve only. 



In the adult age, the shell is irregular, very variable in its form and very inequivalve. 

 The inferior valve is fixed by means of its substance, or rather filling and levelling all 

 the inequalities of the substance on which it rests, so as to solder it there, without, 

 however, in any part losing its external epidermis. 



The buccal region is terminated by a talon, more or less long and irregular, and at 

 the extremity of this talon, always somewhat roughened by marks of growth, the two 

 Anodontiform valves of the young age invariably form a special point. 



This description is illustrated by several figures, which it is to be regretted are not 

 drawn with the care which so interesting a specimen deserved. 



The considerations to be given to this remarkable shell, which M. D'Orbigny 

 thus brings to our notice, are highly important. They involve its position in a 

 natural arrangement, by its affinities, and of course call our attention to the genus 

 Etheria of Lamarck, also an inhabitant of fresh waters, far removed from the sea, on 

 another continent; but which, throughout its whole existence, has belonged to the 

 Dimyaria. The genus Mulleria of Ferussac should be referred to here, it being also 

 a fresh water shell and having but one muscular impression. It is introduced here 

 simply to say that I shall hereafter shew that if M. D'Orbigny's Acostcea be not 

 identical with this shell, described by M. Ferussac in 1823, it will be likely, at least, 

 to be found to belong to the same genus. 



The genus Etheria was first described by Lamarck, in the An. de Museum, and he 

 considers it as marine, and probably affixed to rocks in a deep sea. It excited much 

 interest at the time, as a curious and rare shell. This interest was, however, greatly 

 increased, when M. Cailliaud returned from his expedition to the Blue river, one of 

 the branches of the Nile, and stated that he had brought oysters from its beds, M. 

 Ferussac wrote an able paper on these Etheria brought by Cailliaud, and stated the 

 importance of the fact of their proving to be of fresh water origin, as in a geological 

 point of view, such shells, if found in fresh water deposits of the older strata, might 

 be considered as alternating with marine beds, while in fact they ought to testify to 

 their fresh water origin. These Etheria were found from the first cataracts of the 

 Nile, all the way up as far as Fazool, the highest point reached. 



This Genus {Etheria*) has caused great difference of opinion as to its position in a 



♦The mantle oi Etheria is entirely open, but there is an "anal passage formed by adhesion of the branchiee, as in 

 Unio, and as in the Naiadea there is a large foot." It has prismatic cellular structure.— Dr. Carpenter's Report 

 British Association, 1844, p. 22. 



