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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 type. A 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 rhinocal 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- 



