Bivalve Molluscous Animals, 165 



the case in the genera Venus, Cardium, Tellina, Nucula, Pectunculus, 

 Area, UniOy Anodoiita, Mactra, Donax, Cyclas, it is still vertical, the tubes 

 being above, and the mouth below ; but sometimes it inclines a little 

 to a side. 



Symmetrical shells provided with abyssus which attaches them to the 

 rock, have positions difFering a little from one another. In the Bisso^ 

 area and the Venericardia, they are fixed in such a manner as to give 

 them the same attitude as the Venus in a free state. In the Mytilusy 

 Modiola, and Pinnaj the position is different, the hinge being placed be- 

 low in place of being at the side, and the gaping portion of the valves 

 being above. In this case, nevertheless, the animal is still in the same 

 relative position, for the mouth is always below, and the anus above. 



UNSYMMETRIGAL SHELLS. 



If, a priori, a symmetrical bivalve shell announces a vertical normal 

 position in the direction of the large diameter, we are equally certain 

 that all non-symmetrical bivalve shells have a natural position quite dis- 

 tinct, and analogous among the moUusca, to that of the Pleuronectes 

 with respect to other fishes, that is to say, that the animal, in place of 

 presenting its corresponding portions, or rather the line of separation of 

 the two lobes of the mantle in a vertical direction, does so in a horizon- 

 tal one ; so that non-symmetrical bivalves are in the normal position, 

 relatively to the others, when they repose on their sides. There is no 

 longer in their case, as there is in all symmetrical shells, the distinction of 

 right valve and left valve, but we have always an upper valve and a lower 

 valve. 



With the exception of the Corhula and Pandora, anomalous among the 

 free shells, on account of their irregularity (although their normal posi- 

 tion is vertical), all the other unsymmetrical bivalves are fixed either by 

 means of a byssus, or by the shell itself. 



When they are fixed by a byssus, they are much less irregulari as in 

 the Perna^ Avicula, Malleus, Vulcdla, Pecten, &c., in which a scrupulous 

 examination is sometimes requisite in order to discover the differences 

 between one valve and the other. 



When, on the contrary, the shelVis fixed either to the ground or to sub- 

 marine bodies by the calcareous matter of the shell itself, not only are the 

 superior and inferior valves very unequal, but, moreover, these shells being 

 constrained to conform in their growth to the space which has fallen to 

 their lot, we find them either moulding themselves on the bodies on which 

 they are parasites, or modifying themselves according to the conditions 

 of existence in which they are placed, and thus producing so great an 

 alteration in the form and aspect of different individuals of the same 

 species, that it is necess^'ry to forget altogether the ordinary limits of 

 variations, and to allow a much wider range as to specific characters ; 

 and this is the case with the genera Chama, Spondi/lus, Plicatula, and es- 

 pecially with the Ostrasa and Gryphcea, 



