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Show 20 ZOOLOGY OF THE VOYAGE OF TilE DEAGLE. outer surface of the tooth (fig. 2, Pl. IV.) is slightly concave in the tt·ansverse 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 fiat at the bottom, and from the auterior 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 longitudinal 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 posterior grinders on each side, had a similar structure to the tooth above uescribed. The external layer of enamel is uniformly about half a line in thickness ; it is interrupted 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 crementum, 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 Hydromy~ and Aulacodus, only two on each side. In Lepus, however, we find six on each std~ of the ~pper, and five on each side of the lower jaw. The Toxodon, like the Taptr and H1p?opot~mus, has seven on each side of the upper jaw: the first in each of these spec1es bemg the smallest. It is worthy of notice however that the ?apybara whi:h adheres .to the Rouent type in the number of i~s molars.' presents 111 the. vastl~ mcreased stze, and additional number of component laminre of the postenor grmders • an apll roxt· ma t't on to t h e pach ydermatous character J·U St ad-duced, and the bony palate at th e same t'1 me presents an expansi·O n between these mol :ffi · R ars, 0 e~mg a resemblance to the Toxodon which I have not found in any other odent bestdes the Capybara. The most important de · t' f h R . VIa Ion rom t e odent structure presented by the teeth, occurs m the direc,t ion of th e re fi ecte d 1r 0ld of enamel, and such a deviation FOSSIL MAMMALIA. 21 might have been inferred, even in the absence of the teeth, from the structure of the articular surface, or glenoid cavity for the reception of the condyle of the lower jaw. As the ridge of enamel runs, as above described, in a direction approaching that of the longitudinal axis of the skull, it is obvious that the grinding motions of the lower jaw should be in a proportionate degree in the transverse direction. The glenoid cavity, therefore, instead of being a longitudinal groove, and open behind, as in the true Rodents, is extended transversely, and is defended behind by a broad descending bony process preventing the retraction of the jaw, and showing marks of the forcible pressure to which it was subject. It is worthy of observation that, in the Wombat,-which exhibits the Rodent type of dentition, and, like the Toxouon, has remarkably curved molars, but in an opposite direction,-the condyle of the lower jaw is also extended transversely, and adapted to an articular surface, which admits of lateral motion in the trituration of the food. In the outward span of the zygomatic arches, in which Toxodon deviates from the Rodentia, we may trace a relation of subordinacy to the above structure of the grinding teeth and joint of the lower jaw: the widening of the arches giving to the masseter muscles greater power of drawing the jaw from side to side. The depth of the zygoma bespeaks the magnitude of these masticatory muscles, and the included space shews that the temporal muscles were also developed to a degree, which indicates the force with which the great incisors at the extremity of the jaws, were used; probably, like the canines of the Hippopotamus, to divide or tear up by the roots the aquatic plants, growing on the banks of the streams, which the Toxodon may have frequented. In the Rodentia, the zygoma, though sometimes as deep as in the Toxodon, is generally almost straight, and the space included between it and the cranium is consequently narrow. The zygoma also is placed more forwards in all true Rodents, than in the Toxodon; and, instead of abutting against the posterior alveoli, it terminates opposite the anterior ones. It thus affords such an attachment to the masseter, that this muscle extenus obliquely backwards to its insertion in the lower jaw, at an angle which enables it to act with more advantage in drawing forwarus the lower jaw,-a motion for which the joint is expressly adapted. In many Rodents, also, there is a distinct muscle, or portion of the masseter, which passes through the ant-orbital foramen, which is on that account of large size. In examining the cranium of Toxodon, with reference to this structure, it was found that the antorbital foramen was not larger than might have been expected to give transmission to nerves requisite for supplying with sensibility the large lips, and whiskers with which the expanded muzzle of this remarkable quadruped was probably furnished. Having thus examined the cranium of the Toxodon in its relation, as a |