As connected with the physical distribution of mol luscous animals, we may here offer a few observations On the revolutions which they have experienced. From the oldest secondary rocks to the newest alluvial depo sits, the remains of the hard parts of these animals may be observed. In the newer situations, these parts are less altered than in the older ones, in which they are frequently changed into calcareous spar, clay, flint, or pyrites. The forms exhibited by the remains in the older rocks are different from those in the newer ones, and intimate that they have belonged to races now no longer existing on the surface of the globe. These re mains do not characterize any particular beds or for mations, different beds often producing similar remains, and similar beds containing dissimilar fossils, intimating that the physical and geographical distributions of these animals in the earlier parts of the earth's existence, were regulated nearly by the same Laws which prevail at pre sent. But into this interesting subject it is impossible at present to enter.
The mollnscous animals furnish an agreeable repast to many quadrupeds, birds, and fishes. To man they yield a great deal of palatable and nutricious food. Those which inhabit the sea are held in the highest estimation, while the terrestrial and fluviatile races are generally neglected.
The manner of preparing and preserving the hard parts of the mollusca, has been already given in sufficient detail in the conclusion of our article CONCHOLOGY, to we refer the reader. There is only one method of preserving the softer parts—immersion in spirits of wine. But in the execution of this plan some caution is requisite, otherwise the object will appear a shrivelled shapeless mass. The animal should be permitted to die slowly, that the different parts may become relaxed, otherwise the examination of the form of the body at a future period becomes impracticable. A quantity of the spirits ought to be injected into the stomach or other cavities of the body immediately after death, to prevent putrefaction, as it frequently happens, when the body is immersed in spirits without such precaution, that the viscera become unfit for examination, while the integu ments remain sound.
In the following general view of the different classes into which molluscous animals have been divided, we shall follow the arrangements of M. Cuvier, modified by the systematical labours of M. Lamatck. Frequent reference will be made to species, natives of the British isles, to enable the curious reader to comprehend the re cent improvements which have been effected in this branch of natural history.