12S LEA ON THE GENUS ACOST^A OF D'ORBIGNY. 



becoming free, and subsequently again becoming affixed, but by another manner, soldering 

 or fastening itself to substances on the bottom of rivers. 



There is, too, another very interesting and important consideration in connexion 

 with this singular mollusc. One which seems to me to be likely to change a part at 

 least of the established systematic arrangements. If we admit the theory, that 

 embryonic development will indicate a relative position in the chain of beings, a 

 theory now almost universally accepted, we must place the whole order of the 

 unimusculose molluscs before that of the bimusculose. My attention was called by my 

 friend Dr. Leidy to this consideration. It is one of great interest, and will require 

 the mature attention of zoologists engaged in arranging aqd constructing systems on 

 anatomical differences. 



It will not be uninteresting now to examine the opinions of various distinguished 

 systematists regarding the two' kindred genera Etheria and Mulleria. 



It has been stated before, that Lamarck considers the Etherice to belong to the 

 deep sea. The few specimens which came under his inspection were, no doubt, all 

 more or less imperfect. Of these he made four species, which may be reduced to two 

 if not to one. He supposed them to come from Madagascar and the Indies. When 

 this author published his 6th volume of Animaux Sans Vertebres, in 1819, he had 

 not seen the Mulleria, which was not described by Ferussac until 1823, when he 

 observed it among the Etherice of the celebrated collection of the Duke de Rivoli. In 

 the new edition of Lamarck, by Deshayes, the Mulleria is quoted as a young Etheria 

 transversa, and he refers to Sowerby's figure, but he does not admit Mulleria as a 

 genus. It is evident that M. Deshayes did not believe in Ferussac's Midleria, for, in 

 his note, he says, " the two muscular impressions are always distinct in the old 

 specimens, but that in the young, it sometimes happens, one only can be dis- 

 tinguished. That it was upon an individual in this peculiar condition, that Ferussac 

 established his genus Mulleria, which it is impossible to keep. As to the crenuiations 

 of the hinge, of which M. Ferussac speaks, we have seen in the specimen itself, 

 which this author had in his hands, some small accidental fractures which had 

 taken place, so it appeared to us, as if the shell, being taken with the animal in it, 

 they had separated the valves when cutting the ligament with an edged instrument."* 

 Deshayes' s Lam. Vol. vi. p. 593. 



When M. Ferussac read his memoir, on the genus Etheria, before the Academy of 

 Sciences of France, Messrs. Brongniart and Latreille were appointed to report upon 

 this able contribution. It was in this memoir, published in the Memoirs of the Society 

 of Nat. Hist, of Paris, vol. i. p. 353, M. Ferussac proposed his genus Mulleria. He 

 informs us that on examining the two specimens of Etheria in the Rivoli collection, 

 he was surprised to find that one had only one muscular impression. He says, " Si 

 les Huitres de M. Cailliaud se trouvaient etre des Etheries, cette Etherie se trouve, 



* This was probably the result of the fracture of the ligament. 



