1826.] Mr, Gray on the Genus Hinnita of De France, S^c, 105 



edge, and consequently the right and left valves may be distin- 

 guished in those shells which are provided with only one 

 adductor muscle. Indeed, I only need to speak of those mono- 

 myaires of Lamarck, which have the elastic ligament placed in 

 an internal cavity, as the others may be easily distinguished by 

 the position of their external cartilage, as pointed out in my 

 former remarks. In the whole of these shells, the adductor 

 muscle, and consequently the muscular impression is placed 

 eccentrically, except in the genera Anomia and Placuna. Now 

 by examining the animals, figures of which may be seen in 

 the appendix to Lister, who has given the anatomy of the 

 oyster and scallop, and the plates of Poli models, the mouth 

 will be found to be always placed on the opposite edge, and the 

 anus on the same edge of the shell as the adductor muscles, 

 consequently that edge of the shell to which the muscular 

 impression is nearest, must be its posterior edge, and now if the 

 hinder margin of the shell be placed towards the observer with 

 the hinge edge uppermost, the sides of the animal and shell will 

 correspond with that of the observer. 



I have been thus particular in pointing out this distinction of 

 the valves of these shells, because the observance of this fact 

 will materially assist the natural disposition of the micropodous 

 bivalves, and enable them to be separated into natural families, 

 a division which is now absolutely requisite from the number 

 of genera being so much increased by the great attention 

 which has lately been paid to this interesting study. 



The animals of this groupe, which are attached to the rocks, 

 &c. by a byssus, as the PectenSy have always the groove for the 

 transmission of the byssus, placed under the ear of the right 

 valve ; this character at once distinguishes the Pectinidce, a 

 family which contains the genera Pecten, Anusium, Janira, 

 Neithea, Pallium, Pedum, and Lima. Amongst those genera 

 which are attached to the rocks by the means of the outer sur- 

 face of the shell themselves, which appear to be the typical 

 groupe of micropodous mollusca, the oysters are always fast- 

 ened by the left valves, and the genera Ostrea and Gryphea may 

 therefore form a family under the name of OstreidicB, while the 

 SpondylidcRy which contain the genera SpondyluSj Hinnita and 

 Plicatulay are always attached to the marine bodies by their 

 right valves, and consequently the muscular impressions are 

 placed on the right side of the attached valve, which makes the 

 impression of Hinnita to appear to be placed on the side of the 

 shell opposite to that of the oyster, as described by De France, 

 who appears not to have observed that the difference was only 

 occasioned as in what is erroneously called the reversed 

 chama, by the animals being fastened by the other valve. 



The fact of the conical univalves being considerably altered 

 in form and figure by the substances to wluch they are attached; 



