CONDYLARTHEA. 441 



M. 



Depth at posterior border of third premolar 027 



Depth at third true molar OSD 



Length of inferior dental series , 1'23 



Length of base of third premolar tjl2 



Diameters fourth premolar^ ^"*'^''"P'«t'^^i'"' '^1'* 



( trausverse 009 



Diameters first true molar ^'^"^"■■'l^"'^^'^""'" '^l'^ 



c transverse Oil 



Diameters third true molar ^^"t^^^Pos*"™' 01* 



( transverse Oil 



Width of mandibular condyle 0225 



Besides the general features already described under the head of the 

 genus, the brain of the Phenacodus primoevus displays the following fea- 

 tures : The olfactory lobes are as wide as long, and they diverge, having 

 two external sides. In section they are triangular, presenting an angle 

 downwards. The hemispheres are depressed, and widen posteriorly. They 

 are well separated from each other and from the cerebellum; so much so 

 that it is quite probable that the corpora quadrigemina are exposed. Their 

 outlines are however not distinguishable on the flat surface which connects 

 the hemispheres posteriorly. No further indication of sylvian fissure can 

 be seen in the cast, beyond an entering angle defining tlie lobus hip- 

 pocampi anteriorly. The latter is prominent externally and less so down- 

 wards. There are distinct indications of convolutions. There are three 

 on each side above the sylvian convolution, and a fourth extends from the 

 sylvian upwards and posteriorly below the posterior part of the third or 

 external convolution. The sulci separating the convolutions are very shal- 

 low. The internal and external convolutions unite anteriorly, passing 

 round the exti'emity of the median convolution. The space between this 

 gyrus and the base of the olfactory lobe is only three millimeters. 



The cerebellum is longer than a single hemisphere. Its superior 

 surface is somewhat flattened and descends forwards; the lateral boundary 

 of this face is a projecting edge which rises behind to an angle of the 

 vermis. The posterior face is shorter than the superior, and is vertical. 

 It is separated by a space from a very prominent lateral convolution, wliile 

 the region of the flocculus is concave from the internal form of the ascend- 

 ing portion of the petrous bone. This concavity is open anteriorly. The 

 base of the fifth pair of nerves is below its apex, and that of the sixth 



