LEA ON THE GENUS ACOST^A OF D'ORBIGNY. 127 



natural arrangement. Lamarck, in view of its possessing two muscular impressions, 

 placed it among the Chamacece, and makes it the last genus among the Dimyarice, and 

 of course far removed from Ostrea with one muscular impression. He says it is very 

 singular to find, among the Dimyaria, shells which are inequivalve, irregular and 

 affixed to marine bodies, like Ostrea, Spondijlus, and other MonomyaricB. He then 

 observes, " Ce fait montre que nulle part la nature ne passe brusquement d'un ordre 

 de choses a un autre, sans laisser quelques traces de celui qii'elle abandonne, et 

 meme sans en ofFrir encore quelques-unes au commencement du nouvel ordre qu'elle 

 etablit," vol. 6, p. 90. He says that the Chamacece indicate the proximity of the 

 Monomyarice, and therefore ought to terminate the Dimyarece, so that the Tridacnece 

 would commence the second order. It is very evident from these observations of 

 Lamarck, that had he known the genus Mulkria, he would have placed it at the head 

 of the Monomyarice, as being nearest to Etheria, with which we have seen above he 

 terminates the Dimyarice, and it seems to me fully to sustain that philosophic view, 

 which he so happily expresses in the above quotation. The genus Mulkria must 

 indeed connect the two grand divisions, with a less abrupt separation than he 

 imagined, and it makes the passage still more simple and easy, as it retains more of 

 the traces alluded to than is found in the genus Tridacnea. 



Under the impression that Acostcea, as described by M. D'Orbigny, is of the 

 same genus as Mulkria, Fer., if it be not the same species,* we are struck with the 

 extraordinary and perfect mode by which nature has selected the passage from the 

 Dimyarice to the Monomyarice, M. D'Orbigny has shown us that the Acostcea, in a 

 young state, every way resembles a young Anodonta, furnished with its two adductor 

 muscles, and of course able to enjoy locomotion in its early condition. As it advances 

 in age this power ceases, it becomes an affixed shell, and it no longer has two adductor 

 muscles. It norv belongs to the second order, Monomyaria, having in its progress of 

 maturation abandoned the former order ! In its early condition, slightly removed 

 from its embryonic state, it belonged to the Family ISaiadea, a free mollusc of the 

 first order. In its adult state it belongs to the Family Tridacnea of the second order.f 

 Beino" placed as the first of this Family, its true position, the passage becomes complete. 



If we assume that, in its young state, Acostcea {Mulkria ?) has all the characters of 

 Anodonta, then it would be provided with a byssus, as Dr. Kirtland has shown the 

 Uniones have, when in that condition of early growth. Here, then, we would have 

 a mollusc, first having anchored itself by a filamentous attachment, then probably 



* I have never seen either of the specimens. 



j It is proper to note that M. D'Orbigny in his admirable researches in South America, discovered in the Rio 

 Parana above Corrientes, a most interesting shell, which he found to have the characteristics of the genus Anodonta, 

 except that it was attached by a byssus, and so remained during life. It was small, about one-third of an inch, 

 but in great quantity. For this shell he very properly instituted a new genus, which he named Byssanodonta, and 

 it makes the "nuance" still more easy from the Dimyaria to the Monomyarice. It should be placed as the last of 

 the former. 



