158 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XIV. 



The olfactory bulbs are closely appressed to each other in the me- 

 dian plane, convex above and concave below. They are very slightly 

 overlapped dorsally by the cerebral hemispheres. From the widely 

 flaring lateral borders of the bulb the lateral olfactory tract passes spinal- 

 ward and ventralward along the lateral border of the very large tuber- 

 culum olfactorium. 



The cerebral hemispheres extend backward quite to the cerebellum, 

 these two structures being in intimate contact for the entire width of the 

 body of the cerebellum. There is a very slight divergence of the posterior 

 borders of the hemispheres from the median plane, which is somewhat 

 exaggerated in Figure i (PL XXI), so that even if the meninges were 

 entirely removed from the median longitudinal fissure (as has not been 

 done) , but little if any of the mesencephalon would be visible from the 

 dorsal surface. This is in contrast to the usual marsupial arrangement, 

 for the corpora quadrigemina are in most cases well exposed dorsally. 

 (Petaurus is another exception; see Elliot Smith, '95, p. 168.) 



Superficially the cerebral hemisphere, exclusive of the olfactory 

 bulb, exhibits three chief regions: (i) the dorsal convexity; (2) the 

 lateral convexity, or pyriform lobe; (3) the ventral convexity, or 

 tuberculum olfactorium. 



The dorsal convexity of the hemisphere is purely neopallial; that is, 

 it is non-olfactory cortex. About one-fifth of the distance backward 

 from the frontal to the posterior pole of the hemisphere there is a 

 distinct, though shallow, transverse sulcus which probably represents 

 the sulcus orbitalis of Elliot Smith's descriptions. Otherwise the dorsal 

 convexity is smooth. 



The dorsal convexity is bounded laterally by an imperfectly devel- 

 oped fissura rhinalis. This begins anteriorly as a sharp sulcus in the 

 transverse fissure between the olfactory bulb and the cerebral hemisphere 

 at the dorso-lateral angle of the tuberculum olfactorium (PI. XXI, fig. 2) 

 and extends backward. On the lateral aspect of the hemisphere it is 

 obscurely confluent with the orbital sulcus, and behind this level it 

 disappears in a depressed area on the lateral wall of the hemisphere. 

 Two-thirds of the distance back from the anterior to the posterior pole of 

 the hemisphere this depressed area is slightly deepened, thus marking 

 more precisely the location of the fissura rhinalis in this region; and at 

 the posterior end of the hemisphere there is a wide, shallow notch which 

 marks the posterior end of this fissure (PI. XXI, figs, i and 2). 



The pyriform lobe (lobus piriformis) comprises the larger part of 

 the lateral and ventral aspects of the hemisphere. As we have seen 

 above, it is very imperfectly separated from the dorsal neopallial cortex, 

 though the location of this boundary, the fissura rhinalis, is evident. 



