866 
tertiary deposits, and of about the size of the 
Cheropotamus, ap ined to the same family. 
A third kind of dentition characterizes the 
Hippopotumide, in which the tendency to ex- 
cessive and, as it may be termed, monstrous 
developement of the canine teeth, for which 
the typical Suide are remarkable, affects both 
the canines and the incisors. Of this group 
the only existing representative is the Hippo- 
tamus of the great rivers of Africa. In the 
ippopotamide 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 apparatus hitherto met with among 
Pachydermal Quadrupeds is that of the Toxodon, 
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 
* Tofo, arcus; dvds, dens, 
PACHYDERMATA. 
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, ing an 
uniform diameter, as far as the commencement 
of the alveoli of the ri teeth ; o curve 
which they describe is the segment of a circle, 
the peaiend 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 havi or figure, 
like the incisor teeth of the Sumatran Rhino- 
ceros, or the tusks of the Boar. That they 
were op to teeth of a corresponding struc= — 
ture in the upper jaw is proved by their oblique 
chisel-like cutting edge. to 
The name of Elasmotherium has been given — 
to an extinct Pachyderm with fangless me 
surpassing the Toxodon in size, and of which 
only the lower jaw and its dentition is yet 
known; but the characters of the teeth an 
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 uniqui 
and was discovered in the frozen drift or di 
vium of Siberia. “| 
In the Rhinocerotide, including the pical 
Rhinoceros, the extinct Acerotherium, which 
had no horn, and the equally hornless sm 
existing genus Hyrar, the molar teeth are 
implanted by distinct roots. There are m 
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 0 
these teeth we may discern the same in 
relation to the developement of the horns whiel 
is manifested by the canines of the Ruminat 
Thus the two-horned Rhinoceroses of Af 
which are remarkable for the great lens h 
one or both of the nasal weapons, have 
incisors in their adult dentition, neither h 
that great extinct two-horned species ( RA. tic 
rinus ), the prodigious developement of wh 
horns is indicated by the singular modificatic 
of the vomerine, nasal, and intermaxi 'y bo 
in relation to the firm support of those weapo 
The Sumatran bicorn inoceros Ce 
with comparatively small horns moderate 
developed incisors in both jaws, and the sa 
teeth are present in the nearly allied two-hon 
Rhinoceros called after its discoverer Sehle 
macher. ar 
The incisors are well developed in both 
existing unicorn Rhinoceroses, indi 
and Rh. Sondaicus, but they attain the sensi 
dimensions in the singular extinct hornl 
species, the Rh. incisivus of Cuvier. — 
cae 
* *Enacua, a plate; Onpicy, a beast. 
a 
