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ART. XV. — On the Genus Acostcea of D'Orhigmj, a Fresh Water Lamellihranchia. 



By Isaac Lea. 



The distinguished naturalist, M. D'Orbigny, announced, in the " Revue et 

 Magazine de Zoologie " for Jan., 1851, a new shell of the Family UnionidcB, which 

 he placed between Anodonta and Etheria, and called it Acostaa Guaduasana. It was 

 brought from New Granada by Col. Acosta. In the April number is contained a full 

 and interesting account of this very remarkable shell, with figures in the young and 

 mature states. 



I propose to give some account of this, nearly in the author's words, and then pass 

 to the observations I have to make in relation to this, one of the most curious and 

 interesting of the Molluscs which has ever come under my notice. 



M. D'Orbigny says that among recent discoveries with which travellers and closet 

 naturalists have enriched the domain of Zoology, one of the most remarkable is, 

 without doubt, the singular anomaly of organization, which he then presented to the 

 consideration of the Malacologist. It is in effect a fresh water mollusc, of which the 

 bivalve shell commences, in its first existence, by a resemblance to an Anodonta. In 

 having, like it, two free valves, equal and regular, and having two attaching muscles, 

 which, however, afterwards, has its two valves unequal, irregular, and affixed like 

 Ostrea; and having beside, like that genus, hut one attaching muscle. It is in 

 this view, in its young state, an Anodonta, with the characters of Dimyaria of 

 Lamarck; and in the adult state an Ostrea, furnished with the characters of 

 Monomyaria of this author. M. D'Orbigny says it does not present a deformity, an 

 exceptionable case, but a new genus, which ought to be placed in the family Vnionidce, 

 between the Anodonta and the Etherice. The animal, unfortunately, has not been seen, 

 but the character of the muscular impressions leaves no doubt as to its resemblance 

 to the Aiiodontce. The young shell is free, equivalve, inequilateral, shut ; the ligament 

 is lono- and raised ; the interior, like the AnodontcB, is provided with two attaching 

 muscles, characters determined by the elongate form of the whole. In the interme- 

 diate age, while enlarging, the shell suddenly leaves its regular form, being till now 

 free. It lies on its side the right valve below.* This valve being below, moulds itself 

 upon the body which it touches, extends itself on the substance and fixes itself there. 



•Ferussac observed that Ethericz affixed themselves by the left valve, but that one had been found to be placed on 

 the right valve ; one of the specimens of my collection belongs to this exception. The white, blisterlike portions of 

 the nacre seem not to have been understood. They appear to me to- arise chiefly from the fact that the animal 

 formed most of them to cover the inner end of the tubes. 



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