OF DEXISOX UNIVERSITY. 3T 



corpora quadrigemina, 9 mm., thickness of corpora quadrigemina 

 through the crura, 11.5 mm.; thickness of hemispheres, 21 mm.; 

 length of cerebellum, 15 mm.; width of cerebellum at the flocculi, 

 25.5 mm.; thickness of cerebellum, 13 mm.; length of flocculus, 7 

 mm.; length of medulla, 15.5 mm.; width of medulla, 16 mm.; thick- 

 ness of medulla at pons. 10 mm.; cephalo-caudal width of pons, 5 

 mm.; width of cord at insertion of the third spinal nerve, 7 mm. 



Geomys bursarius. This brain conforms very closely to the usual 

 myomorphic type. The figures presented in Plate I show a close re- 

 semblance to that of a white rat, which is introduced for comparison, 

 the most important differences being in the more closely appressed 

 hemispheres, the rhomboidal cerebellum without projecting flocculi, 

 the form and markings of the medulla, and the general compactness 

 of the brain as a whole. The prominences on the ventral surface are 

 more pronounced than in Erethizon. The general appearance is in- 

 termediate between that of Erethizon and that of the rabbit. 



The Rhinencephalon is of the usual form. The short, club-shaped 

 olfactory lobes project about 4 mm. beyond the hemispheres, and 

 are 3.5 mm. wide and 5.5 mm. long on the ventral surface. 



The Prosencephalon differs but little from that of the rat. Aside 

 from the points already mentioned, the Sylvian fissure presents slight 

 differences, being stronger and produced farther dorsad in the rat. In 

 both cases it crosses the rhinalis. The rhinalis fissure, as in the rat, 

 passes farther laterad than in Erethizon and is not so deep. Tne most 

 noteworthy feature of the ventral surface is the prominent postrhinal 

 lobe. The pyriform also appears more prominent than in Erethizon, 

 partly because the ventral surface is not so flat, but recedes more 

 abruptly at the sides. 



The hippocampus, which is at best a complicated and trouble- 

 some region, is here reduced to very simple terms and it is hoped that 

 the series of transverse sections in Plate III may materially aid in un- 

 derstanding its structure. The first description of the hippocampus 

 in rodents which has come to my notice is that of Stieda.* A con- 

 densed translation of his account of it in the mouse is here inserted 

 for convenience of reference, as follows: The two hemispheres may 

 be compared to a single hollow body, whose cavity is in communica- 

 tion with the third ventricle. This hollow body extends caudad over 



*Op. cit. Zeits. I. wis.s. Zoologie, Bd. XIX. 



