132 THE MAMMALIA OF THE DEEP EIVER BEDS. 



M. M. 



Breadth of lower jaw obliquely back of last molar 073 .075 



Length of symphysis of lower jaw .052 



Height of anterior nares 035 .032 



Breadth of anterior nares 026 .032 



Distance between the supraorbital foramina 016 .024 



Distance between the infraorbital foramina 036 .060 



Length of haid palate .126 



Width of hard palate between first molars .045 



Stylohyal, length ?.050 



Epihyal, length 015 



Ceratohyal, length 019 .... 



Basihyal, width 018 



Tliy rohyal, length 040 



Thyroid cartilage, height 016 



The Brain. Having no brain-cast of the John Day genus Eporeodon prepared, 

 I am unable to compare that of Mesoreodoa with it, and must therefore take the 

 White River Oreodon as a standard. In the latter genus there is a considerable 

 degree of individual variation, both in the shape of the hemispheres and in the num- 

 ber and extent of their convolutions. To some of these types the brain of Mesoreo- 

 don presents a much closer resemblance than to others, but is somewhat more 

 advanced and modernized than any of them. This advance is noticeable in the gen- 

 eral form of the hemispheres, which, though not broader behind than in some speci- 

 mens of Oreodon, are much more so anteriorly, and thus the whole brain is fuller, 

 more rounded and tapers less forward. The hemispheres have also slightly increased 

 in vertical diameter, so that they are no longer exceeded in this dimension by the 

 height of the cerebellum and medulla oblongata. Posteriorly, the two halves of the 

 cerebrum are brought closer together and reach the cerebellum in the middle line, 

 not gaping so as to expose part of the optic lobes, as is the case in the White River 

 genus. They do not, however, appear to overlap the lateral lobes any more exten- 

 sively than in that form. 



The sulci are very nearly the same as occur in some specimens of Oreodon, but 

 they pursue a slightly more sinuous course, which gives an appearance of richer con- 

 volutions. The dorsal surface displays (1) a short and straight lateral fissure, which 

 does not connect anteriorly with the suprasylvian, as is sometimes the case in the 

 White River genus. As regards the latter, Krueg (No. 22) regards this sulcus as 

 the splenial, which by an extreme degree of " supination " is exposed upon the dorsal 

 surface of the cerebrum, as in many of the small artiodactyls now living. But as 

 this fissure does not extend to the medial surface of the hemisphere, this interpreta- 



