ENDOCRANIAL ANATOMY OF FOSSIL MAMMALS 347 
Reighard and Jennings, Jayne, Wiedersheim, Johnston, and 
many others. These have proved useful in certain identifica- 
tions, such as the endocranial blood vessels, nerves, foramina, etc. 
Rodentia 
A single cast represents this primitive mammalian group. It 
has not been positively identified, but it has been regarded as 
‘‘nossibly Paleolagus haydeni Leidy,” which Matthew (’01) refers 
to as the most abundant hare in the White River beds to which 
it is limited. A careful comparison of this cast with Paleolagus 
remains in Yale University Museum by Dr. E. L. Troxell has 
led to the conclusion that it is not an ancient rabbit brain. 
The specimen is shown in figures 1 and 2. The cerebrum 
measures 20 mm. in length; 12 mm. in greatest cerebral width, 
and the olfactory bulbs, which are beautifully preserved, have a 
length of 6 mm. and a combined width of 5 mm. 
The surface of the brain, as in modern rabbits and in the 
Sciuromorpha, is perfectly smooth. The rugosities which appear 
in the form of mineral incrustations may represent the meningeal 
vessels (fig. 1), though I am inclined to think they do not. The 
smooth projection (fig. 1, cn) just anterior to the olfactory bulb 
is evidently a portion of the cast of the nasal chamber. The 
olfactory bulbs themselves resemble in great measure those of 
the modern rabbit. They are smooth, but are shorter and 
broader than in Lepus and in the fossil they are more widely 
separated. The inferior surface is imperfect, so I am not able 
to determine the number of fila olfactoria. 
The only surface marking on the cerebrum is a small sulcus 
(fig. 2), evidently representing the rhinal fissure. This is placed 
well down on the lateral surface, so that the neopallium is exten- 
sive. The cerebrum is more slender than in the modern rabbit 
with the pyriform region more protuberant. The lobes are 
rounded as in the squirrels, not flat as in Paleolagus and modern 
lagomorphs. The cerebellum is imperfectly preserved. The 
base had been partially destroyed by being mounted on a wire 
pedestal. 
