THE CEREBRUM AND OLFACTORIES OF THE OPOSSUM, 
DIDELPHYS VIRGINICA. 
By C. L. Herrick. 
With Plates A. B. and C. 
Material had been collected over a year ago for a study of the 
brain of the opossum, but, for various reasons, the completion of the 
paper was delayed until it now seems best to offer such notes as were 
collected upon the cerebrum in their necessarily incomplete form. 
This has seemed the more desirable in as much as this paper forms a 
portion of a series especially devoted to the histology of the gray mat- 
ter and commissures of the cerebrum in the several groups of verte- 
brates and because it in a sense prepares the way for the delayed 
portion of the rodent paper begun in volume V of this bulletin. 
External Form. 
In most particulars the brain of the opossum resembles that of 
rodents, although the relative size of the cerebrum is less than in any 
rodent .tyP^- ^ larger part of quadrigemina is exposed and the pro- 
portions of the infra-rhinalis to the supra-rhinalis portion of the cere- 
brum is less. In one respect only does the opossum brain approach 
that of carnivora, i. e. in the possession of an apparent homologue of 
the crucial sulcus. But there is good reason to doubt the reliability of 
this homology 
The olfactories are relatively very large and are obliquely attached 
to the crura. They contain, as shown beyond, a considerable mass 
of cortex upon the pes. Longitudinal sections show a strong medio- 
ventral fossa filled by a thickening of the pero, especially the glomer- 
ular layer. The cavity or rhinoccel is very large and connected with 
the lateral ventricle by an oval curved aqueductus cruris. The crura 
are large and exhibit a distinct radix lateralis. 
The cerebrum is pyriform with the caudal portions of the hemis- 
pheres divaricated. The hemispheres may each be divided both mor- 
phologically and histologically into a dorsal and ventral portion sepa- 
