BIVALVIA. 5 
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, but 
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 ; 
Unio and Cyclas 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 see 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 
’ 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 
aérating the branchive, and the latter that which carries off the water after this duty has 
been performed. ‘The animals which are furnished with these prolongations are 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 branchie, and these 
consist usually of four riband-shaped lamella, 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 are covered with an outer pellicle or coating, called 
the epidermis, a material more animalized, that is to say, there is less of lime in its 
Composition, and therefore, under ordinary circumstances, less capable of preservation 
