BULLETIN OF THE LABORATORIES 
the brain cavity as to cause the projecting olfactory lobes to be re- 
flexed. The olfactory bulb is about 6 mm. in diameter, narrowing 
posteriorly into the crus, and is thickly covered on the whole cephalic 
and ventral surface with the roots of the olfactory nerves. 
The Prosencephalo 7 i. The general form is hemispherical, each 
cerebral lobe being flattened on the median and ventral aspects. The 
great development of the front part of the cerebrum is the most strik- 
ing feature of the brain. The ventral surface is unusually flat and 
broad. The pyriform and postrhinal lobes are not high, giving this 
surface a very different appearance from that of the brains of the rab- 
bit or Guinea-pig. 
The dorsal surface of the hemispheres is unconvoluted ; on their 
ventral surface the rhinalis and postrhinalis form a continuous and 
strong fissure running their whole length. The compression m the 
region of the Sylvian fissure so conspicuous in most rodents is here 
reduced to a minimum. No trace of the Sylvian fissure itself is no- 
ticeable on the dorsal aspect ; it is however, described and figured by 
Owen* in the case of Hystrix cristata as well marked, though short. 
But there is on the ventral surface a shallow, though well defined, fis- 
sure running from the chiasm laterad to the rhinalis, where it stops 
abruptly. The pyriform lobe thus sharply defined is about 15 mm. 
wide at the widest part opposite the hypophysis. Other small fissures 
on the ve itral aspect are occasionally present, but very variable. On 
the median surface of the hemisphere the callosum is bounded on the 
dorsal and caudal aspects by the so-called callosal fissure. Passing 
ventrad from the splenium of the callosum the hippocampal fissure is 
well marked. Caudad to the hippocampal fissure is another small fis- 
sure passing caudo- dorsad to a point about as high as the splenium, 
apparently a suggestion of the splenialis. On the olfactory crus are 
two superficial tracts of lighter color; the smaller and shorter passes 
caudad and mesad, the other can be traced caudad and laterad as far 
back as the chiasm. Their relations are discussed in the paragraph 
on the internal topography of the rhinencephalon. The hippocampus 
is essentially as in Geomys, and under that head this region will be 
discussed, as the true relations are more easily traced in that type. 
'^'“Comparative Anatomy and Physiology of the Vertebrates,” Vol. Ill, 
p. no. 
