TERTIAEY VEETEBEATA OE THE EAY&M. 
0 9 
Mm\(TMe . — The sym])liysis of the mandihle (PI, IV. figs. 1, 3, s.) is long, and its 
ventral snrfaee 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 
s]iout-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 ill a young mandible in which the last molar is being cut, the hinder angle 
of the symphysis is beneath the posterior lobe of pm. 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 corouoid ])rocess rises immediately in front of the 
condyle, and in the specimen described its pointed upper end is about G cm. above 
the articular surface. The anterior border of the corouoid is convex and ])asses by a 
gentle curve into the horizontal ramus, its edge dividing into two ridges, which becoun' 
continuous below with the outer and inner edges of the alveolar border respectively , 
so that in this region the mandible is quite unlike that of a I’roboscidean, in which 
the corouoid arises on the outer face of the horizontal ramus in advance of tlu' 
hinder end of the last molar. Tlie jaw further diflers widely from that of a 
Proboscidean in the form of the symj)hysial region and in the presence of a closed 
series of incisor, canine, and cheek-teeth : in both the Proboscidea and in Ikiryther'mm 
the incisors are modified to form procumbent tusks, and there is a long diastema. 
