| ADMEASUREMENTS OF THE CRANIUM OF TOXODON. | feet | inches | lines |
|---|---|---|---|
| Extreme length | 2 | 4 | |
| Extreme breadth | 1 | 4 | |
| Extreme height, (exclusive of the lower jaw) | 10 | ||
| Length of zygomatic process | 1 | 1 | 6 |
| Depth or vertical extent of do. | 6 | ||
| Transverse extent of zygomatic fossa | 6 | ||
| Transverse diameter of cranium between the zygomatic arches | 5 | ||
| Transverse diameter of occipital plane of the cranium | 1 | ||
| From the outside of one condyle to that of the opposite condyle | 8 | 6 | |
| Length of the bony palate | 1 | 6 | |
| Extreme breadth of ditto | 6 | ||
| Breadth of palate at the intermaxillary suture | 2 | 6 | |
| Breadth of palate behind the molar alveoli | 3 | ||
| Longitudinal extent of the molar alveoli | 9 | 6 | |
| Longitudinal extent of the diastema | 5 | 6 | |
| Transverse diameter of posterior nasal aperture | 3 | 9 | |
| Transverse diameter of occipital foramen | 3 | ||
| Transverse diameter of glenoid cavity | 4 | 6 | |
| Antero-posterior do. of ditto | 1 |
DESCRIPTION OF FRAGMENTS OF A LOWER JAW AND TEETH OF A TOXODON.
Found at Bahia Blanca, in latitude 39° on the East coast of South America.
In looking over some fragments of jaws and teeth, forming part of Mr. Darwin’s collection of South American mammiferous remains, and which had been set aside with mutilated specimens referrible to species belonging to the family of Edentata, my attention was caught by the appearance of roots of teeth projecting, in a different direction from the grinders, from the fractured anterior extremity of a lower jaw, and I was induced to examine minutely the structure of the teeth in this specimen, and to search the collection for corresponding fragments. The result was the discovery of portions of the two rami, and the commencement of the symphysis of a lower jaw, containing anteriorly the roots of six incisors, and at least six molars on each side; but as the rami had been fractured through the middle of the sixth alveolus, the number of grinders may have corresponded with those in the upper jaw of the Toxodon.
The most perfect of these fragments is figured in Pl. [V]. figures 1 and 4; figure 2 shows the form of the teeth in transverse section, and the disposition of the enamel upon the grinding surface of the molars on the right side, as restored from a comparison of the fractured teeth in the two rami. From the remains of the symphysis shown at fig. 4, it will be seen that the jaw was remarkably compressed, or narrow from side to side; while the rami (fig. 1.) were of considerable depth, in order to give lodgment to the matrices and bases of grinders enjoying uninterrupted growth.
The pulps of the six incisors in this lower jaw are arranged in a pretty regular semicircle, whose convexity is downwards; the teeth themselves are directed forwards, and curved upwards, like the inferior incisors of the Rodentia. The form and degree of the curvature are shown in the almost perfect incisor (Pl. [V]. fig. 5) which corresponds with the left inferior incisor of the lower jaw, and was found in the same stratum, but belonged to another individual.
These incisors are nearly equal in size: they are all hollow at their base, and the indurated mineral substance impacted in their basal cavities well exhibits the form of the vascular pulps which formerly occupied them. Sufficient of the tooth itself remains in four of the sockets to show that these incisors, like the nearly perfect one (fig. 5), had only a partial investment of enamel; but though in this respect, as well as in their curvature and perpetual growth, they resemble the dentes scalprarii of the Rodentia, they differ in having a prismatic figure, like the inferior incisors of the Sumatran Rhinoceros, or the tusks of the Boar. Two of the sides, viz., those forming the anterior convex and mesial surfaces of the incisor have a coating of enamel, about half a line in thickness, which terminates at the angles between these and the posterior or concave surface. In plate [V]. fig. 4, the enamel of the broken incisors is represented by short lines, showing the direction of its crystalline fibres; the white space immediately within the enamel shows the thickness of the ivory at the base of the tooth, the included gray substance represents a section of the formative matrix or pulp of the tooth, which was of the usual conical form: the inferior broken end of the incisor (fig. 5,) appears to have been distant about one-third from the apex of the pulp.
From the relative position of the bases or roots of these incisors, we may infer that they diverged from each other as they advanced forwards, in order to bring their broadest cutting surfaces into line. That they were opposed to teeth of a corresponding structure in the upper jaw is proved by the oblique chisel-like cutting surface of the more perfect incisor: and it is not without interest to find that the presence of dentes scalprarii at the anterior part of the mouth has not been necessarily limited to Mammalia of small size.
The position of the pulps of these incisors, in close proximity with the anterior grinders, corresponds with the position of the pulps of the incisors in the upper jaw of the Toxodon, and indicates, in conjunction with the size of the pulps, that a considerable extent of the inferior incisors was lodged in the substance of the anterior part of the jaw. It is most likely that no vertically directed tooth would be developed in the part of the jaw so occupied by the curved bases of the incisors, and hence a diastema or toothless space would intervene between the molars and incisors of this lower jaw, as in the upper jaw of the Toxodon.
It is interesting, also, to observe, that as the deviations from the Rodent type, which occur in the cranium of the Toxodon, are the same, in some instances, as those which obtain in the Wombat; so we find a corresponding deviation in the size and relative position of the inferior incisors, which, as in the Wombat, terminate anterior to the molar teeth, instead of extending backwards beyond the last grinder, as in most of the true Rodents. The Capybara presents the nearest approach to this structure, the pulps of the inferior incisors being situated opposite the interspace of the first and second grinders.