OF DENISON UNIVERSITY. 29 



T/ie Dicncephalon. The thalami are larger in Erethizon than in 

 Geomys, the cephalo-dorsal aspect being especially depressed in the 

 latter case. They are somewhat dome shaped above, and each is cov- 

 ered on the dorsal aspect by the hippocampus as by a cap. The 

 corpora geniculata are large and quite obvious superficially. The post 

 geniculatum is more conspicuous in Erethizon ; both are well devel- 

 oped in Geomys. The cinereum is a sub-elliptical body, with the 

 cephalic end, which is bounded by the transverse fibres of the chiasm, 

 flattened. Its lateral portions are not covered by the overlapping 

 pyriform lobe as in the rabbit. The mammillary is unpaired, median, 

 and but slightly elevated. It is separated from the rest of the ciner- 

 eum by a special fissure.. The hypophysis is attached at the centre 

 of the cinereum. It is fungiform. 7 mm. long, 7 mm. wide, and 4 

 mm. high. The stipe passes obliquely caudo-ventrad into the caudal 

 portion of the body of the hypophysis, which is separated from the 

 cephalic portion by a deep fissure on the ventral aspect. The epiphysis 

 is of the usual conical form, 4 mm. high. It is pigmented at the apex. 

 The third ventricle passes up a short distance into the basal portion 

 (Plate I, Fig. 3.) 



Tlie McsenccpJialon is almost completely hidden, the cephalic 

 members by the hemisphere, the caudal members by the cerebellum 

 The cephalic members are longer, the caudal members wider and 

 higher. This arrangement is very different from that of Arctomys, 

 in which the cephalic members are much larger and occupy the posi- 

 tion of both members in these types. The caudal members are there 

 crowded back to a position almost ventrad of the cephalic (cf. Vol. 

 V, Plate XIX, Fig. 1.) 



The crura cerebri stand out with great distinctness, being exposed 

 to a point nearly as far cephalad as the attachment of the hypophysis. 

 They are divaricated both cephalad and caudad, giving rise to a de- 

 pression which is hour-glass shaped, the posterior area perforata, lying 

 between the cinereum and the pons. 



The Epericephalon. The cerebellum is subellipsoidal. The floc- 

 culus is rather large, but projects scarcely at all beyond the lateral lobe 

 of the cerebellum, nor is it separated from this lobe by bony partitions, 

 as in most rodents. It lies in a very shallow cavity of the skull in 

 (lose contact with the adjacent parts of the cerebellum. 



The Metencephalon. The medulla is obovate, about as long as 

 wide. On the ventral surface the pons and pyramids are strongly 



