FOSSIL MAMMALIA. 



69 



This fossil was discovered in a bed of partly consolidated gravel at the base of 

 the cliff called Punta Alta, atBahia Blanca in Northern Patagonia : it consists of 

 the lower jaw with the series of teeth entire on both sides : but the extremity of the 

 symphysis, the coronoid and condyloid processes, and the angular process of the 

 left ramus, are wanting. The teeth are composed, as in Bradypus, Megatherium 

 and Megalonyx, of a central pillar of coarse ivory, immediately invested with a 

 thin layer of fine and dense ivory, and the whole surrounded by a thick coating 

 of cement. 



In the fig. 5, PI. XVII., the fine ivory is represented by the white striated 

 concentric tract on the grinding surface of the teeth ; it is of a yellowish-white 

 colour in the fossil, and stands out, as an obtuse ridge, from that surface : both 

 these conditions depend on the large proportion of the mineral to the animal con- 

 stituent in this substance of the tooth. The external layer of the cement presents 

 in the fossil the same yellowish-brown tint as the bone itself, which it so closely 

 resembles, both in intimate structure and in chemical composition ; the internal 

 layer next the dense ivory is jet black, indicating the great proportion of animal 

 matter originally present in this part. The central pillar of coarse ivory, which, 

 from its more yielding texture, has been worn down into a hollow at the triturating 

 surface of the tooth, also presents, as a consequence of the less proportion of the 

 hardening phosphates, a darker brown colour than the external layer of the 

 cement, or the bone itself. 



The teeth are implanted in very deep sockets ; about one-sixth only of the 

 last molar projects above the alveolus ; the proportion of the exposed part of the 

 tooth increases as they are placed further forwards. The implanted part of each 

 tooth is simple ; preserving the same size and form as the projecting crown, and 

 presenting a large conical cavity at the base, indicative of the original persistent 

 pulp, and perpetual growth of these teeth. 



The extent of the whole four alveoli is four inches, eight lines ; the length of 

 the jaw from the angle to the broken end of the symphysis is seventeen inches 

 and a half;* from the figures it will be seen that only a small proportion of the 

 anterior part of the jaw is lost, so that we may regard the dentigerous part of the 

 jaw as being limited to about one-fourth of its entire length ; the alveoli being 

 nearly equidistant from the two extremities. The first and second teeth, counting 

 backwards, are separated by an interspace of rather more than three lines ; that 

 between the second and third is one line less ; the third and fourth are rather 

 more than aline apart: from the oblique position, however, of the three hinder 

 teeth the intervals between them appear in a side view, as in fig. 1, PI. XIX., to 

 be less than in reality, and the third and fourth teeth seem to touch each other. 



* If the lower jaw of Mylodon Harlani, bears the same proportion to its teeth as does that of Afylodon 

 DarwmiL it must be about two feet in length. 



