BIVALVIA. 



enormous thickness, highly calcareous, and with only a small cavity for its inhabitant, 

 while in others the shell is nearly corneous, and in some the soft parts, as they are 

 called, constitute almost the entire animal, the mantle having but a very thin coating of 

 calcareous matter. 



Marine shells, as a general rule, are thicker than those which inhabit fresh water, bul 

 in both the variation is occasionally excessive. Ostrea and Pholadomya may be cited 

 as examples of the extremes of thickness and tenuity in the case of marine Bivalves ; 

 Unto and Ci/clas in those of fresh water. In all these cases, solidity or tenuity of sub- 

 stance does not appear to have been regulated in the animal solely by the want of a 

 protective covering as a preservative to its specific existence. 



Fresh water Bivalves, like the Terrestrial air-breathing Univalves, do not exhibit the 

 great specific variation that we sec in marine animals of the same class. We might 

 naturally expect this to be so in regard to shells inhabiting fresh water, when so small 

 a space is occupied by these animals in comparison with that on which their marine con- 

 geners live, but why the land Pulmonata should in specific enumeration be inferior to 

 other Mollusca is not by any means satisfactorily explained ; the bands round the 

 coast lines which contain nearly the whole of marine Molluscan life being far more 

 limited in their dimensions than the feeding-ground occupied by the Pulmonata. which 

 may be taken as the chief part of the land area generally. 



The headless animals which compose this group, or the division of it called the 

 Dimyaria, are nourished and sustained by two siphonal tubes, formed by a prolongation 

 of the mantle, the one inhalent, and the other exhalent; the former being that through 

 which the water containing the particles of nutrition is conveyed to the mouth, and for 

 aerating the branchiae, and the latter that which carries off the water after this duty has 

 been performed. The animals which are furnished with these prolongations arc necessarily 

 supplied with muscles for their extension, as also for their retraction ; and as a considerable 

 space is required for the play of these tubes, an impression is generally formed by the 

 retractor muscles upon the interior of that part, of the valve, indicating the length or 

 extent to which they are or have been capable of protrusion, and the depth of the sinus 

 in general corresponds with the presumed extent of the siphons. 



Bivalves are all aquatic, and breathe entirely by means of gills or branchiae, and these 

 consist usually of four riband-shaped lamellae, two of them attached to each lobe of the 

 mantle ; water, therefore, is in their case necessary to sustain life ; a few species, however, 

 appear to be able to retain a sufficient quantity of moisture to enable them to live for a 

 considerable time out of water. Shells often acquire an increase of material where there 

 is a superfluity of lime within their reach, and become too ponderous for any apparent 

 requirements of the animal. 



Most shells in the living state arc covered with an outer pellicle or coating, called 

 the epidermis, a material more annualized, that is to say, there is less of lime in its 

 composition, and therefore, under ordinary circumstances, less capable of preservation 



