A third kind of dentition characterizes the Hippopotamick, in which the tendency to ex cessive and, as it may be termed, monstrous developement of the canine teeth, for which the typical Suidm are remarkable, affects both the canines and the incisors. Of this group the only existing representative is the Hippo potamus of the great rivers of Africa. In the Hippopotamidw the implanted base of each of the incisive and canine teeth is simple and excavated for a large persistent matrix, which causes their perennial growth by constantly adding materials at the base of each to replace what is worn from their abraded extremities. The direction of the abraded surfaces is in part provided for by the partial disposition of the enamel ; in the upper median incisor this is laid upon the fore and outer part of the tooth, while in the lateral incisor there is a narrow strip of enamel along the convex side of the tooth. The enamel is soon worn away from the crowns of the lower incisors, but it is per sistent in the canines, where it extends to the end of the implanted base; in the upper canine being laid upon the posterior and outer, but not on the fore part, whilst its position is reversed upon the inferior canine.
The extinct genera of Hippopotamoid Pachy derms at present discovered are the Herapro tadon, the Merycopotamus, and the Anthraco therium.
Perhaps one of the most singular forms of 'the dental appamtus hitherto met with among Pachydermal Quadrupeds is that of the Torodon, a large extinct genus, represented by two spe cies both equalling the Hippopotamus in size, whose remains have been discovered by Mr. Darwin and M. de Angelis in the recent tertiary deposits of South Arnerica. The teeth of the Toxodon consisted of molars and incisors, separated by a long diastema or toothless space. In the upper jaw the molars were fourteen in number, seven on each side, and the incisors four, which latter differ in their proportions in the two species. In the lower jaw there vvere six incisors and twelve molars.
All the molar teeth are long and curved and without fangs, as in the Wombat and most of the herbivorous species of the Rodent order: in existing races, however, with curved grinders, as the Aperea or Guinea-pig, the concavity of the upper grinders is directed outwards, the fangs of the teeth of the opposite sides diverging as they ascend in the sockets ; but in the Toxodon the convexity of the upper grinders is outwards, as in the Horse, but with so much greater curvature that the fangs converge and almost meet at the middle line of the palate, forming a series of arches capable of resisting great pressure. It was this structure which suggested to Professor Owen the generic term conferred by him upon this most remarkable extinct Mainmal.4 Of the upper incisors there are two small ones situated in the middle of the front of the intermaxillaries, and exterior to these two large ones, in close contig,uity with the small incisors, which they greatly exceed in size.
The sockets of the two large incisors extend, backwards in an arched form, preserving an uniform diameter, as far as the commencement of the alveoli of the molar teeth ; the curve which they describe is the segment of a circle, the position, form, and extent of the sockets being such as are only found in those of the corresponding teeth of the Rodentia among existing Mainmalia; and it may likewise be inferred that the pulp which formed them was persistent, and that the growth of those incisors, like those of the Rodeotia, continued through out life. The six lower incisors were all of nearly equal size, hollow at their bases, and partially coated with enamel, like the " dentes scalpmrii " of the Rodentia ; they differed, how ever, from these in having a prismatic figure, like the incisor teeth of the Sutnatran Rhino ceros, or the tusks of the Boar. That they were opposed to teeth of a corresponding struc ture in the upper jaw is proved by their oblique chisel-like cutting edge.
The name of Elasmotherium has been given to an extinct Pachyderm with twigless molars, surpassing the Toxodon in size, and of which only the lower jaw and its dentition is yet known ; but the chamcters of the teeth are sufficiently remarkable, owing to the beautiful undulating folds into which the enamel is thrown, a circumstance from which the name of the genus is derived.* The original jaw, preserved in the Museum of Moscow, is unique, and was discovered in the frozen drift or dilu vium of Siberia.
In the Rhinoceroticke, including the typical Rhinoceros, the extinct Acerotherium, which had no horn, and the equally hornless small existing genus Hyrax, the molar teeth are implanted by distinct roots. There are no canines ; and as to the incisors the species vary, not only in regard to their form and proportions, but also their existence, and in the varieties of these teeth we may discern the same inverse relation to the developement of the horns which is manifested by the canines of the Ruminants. Thus the two-horned Rhinoceroses of Africa, which are remarkable for the great length of one or both of the nasal weapons, have no incisors in their adult dentition, neither had that great extinct two-horned species ( Rh. ticho rinus), the prodigious developement of whose horns is indicated by the singular modifications of the vomerine, nasal, and intermaxillary bones in relation to the firm support of those weapons. The Sumatran bicorn Rhinoceros combines with comparatively small horns moderately developed incisors in both jaws, and the same teeth are present in the nearly allied two-horned Rhinoceros called after its discoverer Schleier macher.