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ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



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 



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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 con- 

 siderable 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 semi-circle, 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 



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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 ter- 

 minates 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 



