PACHYDERMATA. 



tertiary deposits, and of about the size of the 

 Chaeropotamus, appertained to the same family. 



A third kind of dentition characterizes the 

 Hippopot amid/z, in which the tendency to ex- 

 cessive and, as it may be termed, monstrous 

 developement of the canine teeth, for which 

 the typical Suidae 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 

 HippopotamidtE 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 Hexapro- 

 tadon, the Merycopotamus, and the Anthraco- 

 tlierium. 



Perhaps one of the most singular forms of 

 the dental apparatus hitherto met with among 

 Pachydermal Quadrupeds is that of the Tojcodon, 

 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 America. 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 were 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 Mammal.* 



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 contiguity 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 Mammalia; 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 Rodentia, 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 

 scalprarii " of the Rodentia ; they differed, how- 

 ever, from these in having a prismatic figure, 

 like the incisor teeth of the Sumatran 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 Elasmothcrium has been given 

 to an extinct Pachyderm with fangless molars, 

 surpassing the Toxodon in size, and of which 

 only the lower jaw and its dentition is yet 

 known ; but the characters 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 Rhinocerotidtf, including the typical 

 Rhinoceros, the extinct Acer other ium, which 

 had no horn, and the equally hornless small 

 existing genus Hyra.r, 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. 



The incisors are well developed in both the 

 existing unicorn Rhinoceroses, Rh. Indicus 

 and Rh. Sondntcus, but they attain their largest 

 dimensions in the singular extinct hornless 

 species, the Rh. incisivus of Cuvier. In the 



To'ov, arc us ; ofovf, dens. 



, a plate ; 



a boast. 



