22 TEETIAET VEETEBEATA OF THE FAYtM. 



Mandible. — The symphysis of the mandible (PI. IV. figs. 1, 3, s.) is long, and its 

 ventral surface is divided into two regions — an anterior rounded portion sloping back 

 from the incisor alveoli, and a posterior portion which is flattened and is in the same 

 line as the ventral surface of the lower border of the horizontal rami, making an obtuse 

 angle with the anterior region. In the middle of this flattened portion there is a small 

 rugose tuberosity for the attachment of muscles, and the dorsal surface forms a narrow 

 spout-like channel. The length of the symphysis seems to have increased with age, 

 at any rate the relative position of its posterior angle with regard to the teeth alters. 

 Thus in a young mandible in which the last molar is being cut, the hinder angle 

 of the symphysis is beneath the posterior lobe of ^m. 4, while in another fully adult 

 it is beneath m. 2 : there seems, however, to be considerable individual variation in 

 this matter irrespective of age. The horizontal ramus is comparatively narrow from 

 above downwards: it widens out a little beneath the last premolar, and gradually 

 narrows beneath the molars. The alveolar border is slightly convex from before 

 backwards, and its inner edge, at least in the molar region, is higher than the outer. 

 The ventral border is slightly convex from before backwards, and strongly so from side 

 to side. The inner faces of the rami are nearly flat, the outer convex. The two 

 rami are separated from one another by a narrow interval, and are nearly parallel as 

 far back as the last molars, but behind this they diverge considerably. The single 

 mental foramen is beneath the fourth premolar. 



The angular region {a.) is separated from the ventral border of the horizontal ramus 

 by a slight concavity : it is rounded and its posterior edge is thickened so as to form a 

 broad surface, looking backwards and somewhat outwards. Superiorly it is bounded 

 by a sharp angle which projects considerably behind the condyle, from which it is 

 separated by a concave border about 8 cm. long. 



The condyle (cd.), which is elongated from side to side, is set somewhat obliquely, 

 and is slightly convex from side to side, rather more so from before backwards. 

 It is supported on either side by a rounded thickened ridge, and in some cases it is 

 divided by a slight median notch into an outer and an inner lobe, which are subequal. 

 The posterior border of . the coronoid process (c.p.) rises immediately in front of the 

 condyle, and in the specimen described its pointed upper "end is about 6 cm. above 

 the articular surface. The anterior border of the coronoid is convex and passes by a 

 gentle curve into the horizontal ramus, its edge dividing into two ridges, which become 

 continuous below with the outer and inner edges of the alveolar border respectivelv , 

 so that in this region the mandible is quite unlike that of a Proboscidean, in which 

 the coronoid arises on the outer face, of the horizontal ramus in advance of the 

 hinder end of the last molar. The jaw further differs widely from that of a 

 Proboscidean in the form of the symphysial region and in the presence of a closed 

 series of incisor, canine, and cheek-teeth : in both the Proboscidea and in JBarytherium 

 the incisors are modified to form procumbent tusks, and there is a long diastema. 



