FOSSIL MAMMALIA. 19 



small incisors, in the hare and rabbit, being so placed immediately behind the large 

 pair, as to receive the appulse of the single pair of incisors in the lower jaw. 



In the Toxodon the position of the incisors, in the same transverse line, might 

 lead to the inference, that they were opposed by a corresponding number in the 

 lower jaw ; but the numerous examples of inequality, in the number of incisors, in 

 the upper and lower jaws of existing mammalia, forbid any conclusion on this 

 point.* The sockets of the small mesial incisors of the Toxodon (s s, PL III.) gra- 

 dually diminish in size, as they penetrate the intermaxillary bones, and we may, 

 therefore, infer that the pulp was gradually absorbed in the progress of their de- 

 velopment ; and that, like ordinary incisors, their growth was of limited duration, 

 and their lodgment in the jaw effected by a single conical fang. 



I may observe, that the formation of a fang is the necessary consequence of 

 the gradual absorption of the matrix or pulp of a tooth ; for the pulp continues, as 

 it diminishes in size, to deposit ivory upon the inner surface of the cavity of the 

 tooth from which it is receding, and the tooth or fang thus likewise progressively 

 diminishes in size. The formation of the socket proceeds uninterruptedly, and 

 the bone encroaching upon the space left by the tooth, closely surrounds the 

 wasting fang, and affords it a firm support ; and thus an inference may be drawn 

 from the form of the socket alone, as to whether the tooth it contained had or had 

 not one or more conical fangs, and consequently whether its growth was temporary 

 or uninterrupted. 



Applying this reasoning to the molar teeth of the Toxodon, we infer that 

 their growth, like those of most of the Phytiphagous Rodents, of the Megathe- 

 rium and Armadillo, was perpetual, because their sockets are continued of uniform 

 size from the open to the closed extremity ; and the molar tooth which is preserved 

 proves the accuracy of the deduction, inasmuch as its base is excavated by a 

 large conical cavity for the lodgment of the pulp, the continued activity of which 

 was the compensation here designed to meet the effects of attrition on the opposite 

 or grinding surface of the tooth. 



The molar tooth discovered by Mr. Darwin in the banks of the Tercero, not 

 only belonged to the same species as the skull under consideration, but to an in- 

 dividual of the same size ; it fits exactly into the socket next to the posterior one 

 of the right side. The figures subjoined of this molar tooth (Fig. 3, PL I.; figs. 2 

 and 3, PL IV.) almost preclude the necessity of a description. The transverse 

 section of the tooth gives an irregular, unequal sided, prism ; the two broadest 

 sides of which converge to the anterior angle, which is obtusely rounded. The 



* This was written before an examination of the fragment of a lower jaw, forming part of Mr. Darwin's 

 collection of Fossil Remains, had led me to suspect that it was referrible to the genus Toxodon ; should this sus- 

 picion prove correct, the four unequal incisors of the upper jaw are opposed to six equal sized ones in the 

 lower. 



