The animals of this class differ in the following general circumstances. The greater number have nails or claws upon their extremities ; some have their extremities terminating in hoofs, while a few have them formed like fins. These three divisions may be subdivided, the two first according to the number and situation of their teeth, and the last according to the situation of their nostrils. Thus, of the mammalia with nails or claws, some have three sorts of teeth, and of these mammalia, one family has thumbs on the atlantal extremities only, as man ; others have thumbs on both atlantal and sacral extremi ties, as the quadrumana; while a third family has no separate thumbs or great toes on the atlantal extremi ties, as the cheiroptera, that have membranes, extended between the divisions of at least their atlantal extremi ties ; the filantigrada, that have no separate thumbs, and rest on the ground with the whole sole of the foot ; the carnivora, that have no separate thumbs, and rest on the ground only with their toes ;.and the nedimana, that have separate great toes on the sacral extremities. A se cond subdivision of the mammalia, with nails or claws, wants at least one sort of teeth, and of these there are three families, viz. the rodentia, that want only the ca
nine teeth ; the tardigrada, that want only the cutting teeth ; and the edentata, that want both the cutting and canine teeth.
Of the mammalia with hoofs, some have more than two toes, or more than two hoofs, as the pachydermata; others have two toes and two hoofs, as the ruminantia; and a third family has only one toe and one hoof, as the solifieda.
Of the mammalia with feet formed like fins, the ani mals of one family have their nostrils at the extremity of the muzzle, as the tribes of phoca and trichecus; while those of another family have them seated in the coronal part of the skull, near the frontal bone ; as the cetacca.
The above classification of the subdivisions of the mammalia is, with the exception of the two last, that of Cuvier; but, as it is not probably familiar to most of our readers, we shall arrange the subdivisions in the form of a table, with the synonymes of Blumenbach and Linne.
The natural history of these animals will be given in the present work, under the articles MAMMALIA and CETEOLOGY.