The incisors are well developed in both the existing unicorn Rhinoceroses, Rh. Indicus and Rh. Sonclaicus, but they attain their largest dimensions in the singular extinct hornless species, the Rh. incisivus of Cuvier. In the adult Rhinoceros no traces of canine teeth are visible, but Professor Owen succeeded in de tecting their existence in a rudimentary condi tion in the mature fcetus of the Rhinoceros Indicus, although both the teeth and their sockets disappear at a very early age.
The vast hiatus which in the series of existing Mammals divides the Rhinoceros from the Tapir, and this from the Elephant, was once filled up by interesting transitional species, which have long become extinct, such as the Pakeotherium and the Illaerauchenia, the Lophiedon, Coryphodon, and others requiring no particular notice in this place. But that most extraordinary of extinct Pachyderms, the Dinotherium, must not be so lightly passed over, inasmuch as its dentition appears to have been quite unique in character, as may be seen on reference to fig. 478, which represents the lower jaw of this gigantic quadruped. From this it will be seen that the molar and premolar teeth resemble in some respects those of the Mastodon ; but the great peculiarity of the Dinotherium exists in its tusks projecting from the lower jaw. These tusks are two in number, implanted in the prolonged and deflected syln physis of the lower jaw, in close contiguity with each other, and having their exserted crown directed downwards and bent backwards, while their deeply implanted base is excavated by a wide and-deep conical pulp cavity, like the tusks of the Mastodon and Elephant. No such tusks nor germs of such have yet been discovered in the upper jaw, so that it is highly probable that this gigantic Pachyderm was of aquatic habits, like the Hippopotamus, and that its tusks served to detach and tear up by the roots the aquatic plants upon which it fed, as well as for weapons of defence or combat.
No family of Mammalian Quadrupeds has suffered more from the destructive operations of time than that of the Proboscidian Pachy dermata.. Two species only, the Indian and the African Elephants, continue to represent this type in the Mammalian series of the present day ; whilst those that manifested the modifications of the dental system which gra dually reduce the complexity of the Elephantine dentition to the comparative simplicity of that of the Dinothere and Tapir, have long since
.been blotted out of the series of living beings. Of these the gigantic Mastodons are the most conspicuous — animals nearly allied to the existing Elephants, but differing from them in the construction of the grinding surfaces of their molar teeth, which had their crowns studded with conical eminences more or less resembling the teats of a cow—whence the generic name is derived.* In addition to these grinding teeth the Mastodons were provided with two enormous tusks, resembling those of the Elephant, implanted into the intermaxillary bones of the upper jaw ; and moreover those Alastodons with the more simple and typical molar teeth likewise manifest the Dinotherian character in having tusks in the lower jaw of the adult male and in the young of both sexes.
The dentition of the Elephant, the sole surviving genus of the great Proboscidian family, consists of two long tusks, one situated in each of the intermaxillary bones, and of large and complex molars in both jaws. Of the latter there is never more than one wholly, or two partially in place and use on each side at any given time ; for like the molars of the Mastodons, the series is continually in progress of formation and destruction, of shedding and replacement, and in the Elephants all the grinders succeed one another horizontally, from behind fonvards, none being displaced and replaced by vertical successors or premolars.
The total number of teeth developed in the Elephant Professor Owen believes to be 2 . 2 6 . 6 Incis. — Molars — = 28, 0 . 0 .
the two large pertnanent tusks being preceded by two small deciduous ones, and the number of molar teeth which follow each other being at least six ; but Mons. Corse was of opinion that this replacement of teeth is repeated at least eight times in the Indian Elephant, which would consequently have thirty-two teeth suc cessively taking their respective places in the jaws.
The deciduous tusk makes its appearance beyond the gum between the fifth and seventh month; it rarely exceeds two inches in length, and is about a third of an inch in diame ter at its thickest part, where it protrudes from the socket; the fang is solidified, and contracts to its termination, which is commonly a little bent and is considerably absorbed by the time the tooth is shed, which takes place between the first and second year.