20 



ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



outer surface of the tooth (fig. 2, PL IV.) is slightly concave in the transverse 

 direction, but undulating, from the presence of two slight convex risings which 

 traverse the tooth lengthwise. The inner surface presents at its anterior part a 

 slightly concave surface, and posteriorly two prominent longitudinal convex ridges, 

 separated by a groove which is flat at the bottom, and from the anterior angle of 

 which the reflected fold of enamel penetrates the substance of the tooth, advancing 

 obliquely forwards, rather more than halfway across the body of the tooth. A Ion- 

 gitudinal ridge of bone projects from the internal side of the socket, and fits into the 

 groove above mentioned, and as a corresponding ridge exists in all the sockets of 

 the grinders, save the two anterior small ones, we may infer that the five posteri 



grinders on each side, had a similar structure to the tooth above described. The 

 external layer of enamel is uniformly about half a line in thickness ; it is inter- 

 rupted for the extent of nearly three lines at the anterior angle, and for more than 

 double that extent at the posterior part of the tooth, which is consequently worn 

 down much below the level of the rest of the grinding surface. Where the ivory is 

 thus unprotected by the enamel, it has a coat of csementum, which also fills up 

 the small interval at the origin of the reflected fold of enamel. On the grinding 

 surface of the entire tooth, and on the fractured ends of the mutilated molars, the 

 component fibres, or tubules, of the ivory, are readily perceptible by the naked 

 eye, diverging from the line which indicates the last remains of the cavity of the 

 pulp of the tooth, as it was progressively obliterated during growth. 



Although the complication of the grinding surface by the inflection of simple 

 or straight folds of enamel is peculiarly characteristic of the Rodent type, we must 

 regard the number of molar teeth, and their diminution of size as they advance 

 towards the anterior part of the jaw, in the Toxodon, as indicative of a deviation 

 from that order, and an approach to the Pachyderms. The common number of 

 grinders in the upper jaw of Rodent animals is eight, four on each side. In some 

 genera, as Lemmus, Mus, Cricetus, there are only three on each side, and in Hy- 

 dromys and Aulacodus, only two on each side. In Lepus, however, we find six on 

 each side of the upper, and five on each side of the lower jaw. The Toxodon, like 

 the Tapir and Hippopotamus, has seven on each side of the upper jaw: the first in 







each of these species being the smallest. It is worthy of notice, however, that the 

 Capybara which adheres to the Rodent type in the number of its molars, presents 

 in the vastly increased size, and additional number of component laminae of the 

 posterior grinders, an approximation to the pachydermatous character just ad- 

 duced, and the bony palate at the same time presents an expansion between these 

 molars, offering a resemblance to the Toxodon which I have not found in any other 

 Rodent besides the Capybara. 



The most important deviation from the Rodent structure presented by the 

 teeth, occurs in the direction of the reflected fold of enamel, and such a deviation 



