The new tooth disembarrasses itself of the cylindrical base of its predecessor, with which it is sheathed, by maintaining the excitement of the absorbent process so long as the cement of the old fang retains any vital connection with the periosteum of the socket ; but the frail remains of the old cylinder, thus re duced, are sometimes lifted off the socket upon the crown of the new tooth, as in fig. 573. b, when they are speedily removed by the action of the jaws. This is, however, the only part of the process which is immediately produced by mechanical force : an attentive observation of the more important previous stages of growth, teaches that the pressure of the growing tooth operates upon the one to be displaced only through the medium of the vital absorbent action which it has excited.
Most of the stages in the development and succession of the teeth of the crocodiles are described by Cuvier with bis wonted clearness and accuracy ; but the mechanical explanation of the expulsion of the old tooth, which Cuvier adopts from M. Tenon, is opposed by the disproportionate smallness of the hard part of the new tooth to the vacuity in the old one, and by the fact that the matter im pressing— viz. the uncalcified part of the walls of the tooth-matrix—is less dense than the part impressed.
No sooner has the young tooth penetrated the interior of the old one, than another germ begins to be developed from the angle between the base of the young tooth and the inner alveolar process, or in the same relative position as that in which its immediate pre decessor began to rise, and the processes of succession and displacement are carried on, uninterruptedly, throughout the long life of these cold-blooded carnivorous reptiles.
From the period of exclusion from the egg, the teeth of the crocodile succeed each other in the vertical direction ; none are added from behind forwards, like the true molars in Mammalia. It follows, therefore, that the number of the teeth of the crocodile is as great when it first sees the light as when it has acquired its full size; and, owing to the rapidity of the succession, the cavity at the base of the fully-formed tooth is never con solidated.
The fossil jaws of the extinct Crocodilians demonstrate that the same law regulated the succession of the teeth, at the ancient epochs when those highly organised reptiles prevailed in greatest numbers, and under the most varied generic and specific modifications, as at the present period, when they are reduced to a single family, composed of so few and slightly varied species as to have constituted in the system of Linnmus a small fraction of his genus LACERTA.
Dental System of Mammals.
The class Mammalia, like that of Reptilia and Pisces, includes a few genera and species that are devoid of teeth ; the true ant-eaters (Myrinecoplzaga), the ant-eaters or Pan golins (Mann), and the spiny monotrematous ant-eater (Echidna), are examples of strictly edentulous Mammals. The Ornithorhynchus has horny teeth, and the whales (Baleen and BalTnoptera) have transitory embryonic cal cified teeth*, succeeded by whalebone sub stitutes} in the upper jaw. Horny processes analogous to, perhaps homologous with, the lingual and palatal teeth in fishes, are present in the Echidna.
The female Narwhal seems to he eden tulous, but has the germs of two tusks in the substance of the upper jaw-bones; one of these becomes developed into a large and conspicuous weapon in the male Narwhal, and, accordingly, suggested to Linnmus the name, for its genus, of Ilionodon, meaning single tooth : but the tusk is never median, like the truly single tooth on the palate of the Myxine ; and occasionally both tusks are developed in the Narwhal. In another Ceta cean, the great Bottle-nose, or Hyperoodon, the teeth are reduced in the adult to two in number, whence the specific name H. bidens, but they are confined to the lower jaw. The sharp-nosed dolphin (Ziphius) has also but two teeth, one in each ramus of the lower jaw ; and this is, perhaps, a sexual character. The Delphinus griseus has five teeth on each side of the lower jaw ; but they soon become reduced to two. Amongst the marsupial ani mals, the genus Tarsipes is remarkable for the paucity as well as minuteness of its teeth.
The elephant has never more than one entire molar, or parts of two, in use on each side of the upper and lower jaws ; to which are added two tusks, more or less developed, in the upper jaw.
Some Rodents, as the Australian Water rats (Hydronzys), have two grinders on each side of both jaws ; which, added to the four cutting teeth in front, make twelve in all : the common number of teeth in this order is twenty ; but the hares and rabbits have twenty eight teeth. The sloth has eighteen teeth. The number of teeth, thirty-two, which cha racterises man, the apes of the old world, and the true ruminants, is the average one of the class Mammalia ; but the typical number is forty-four.