MAY, 1921. BRAIN OF C^ENOLESTES HERRICK 159 



Antero-ventrally it is separated from the tuberculum olfactorium by a 

 very sharp fissura endorhinalis. The ventral surface of the pyriform 

 lobe shows two shallow depressions separated by an elevated ridge run- 

 ning obliquely backward and lateralward from the posterior margin of 

 the tuberculum olfactorium. Further information regarding the internal 

 structure is desirable before the signification of this sculpturing is ex- 

 plained. It may be due merely to the conformation of the brain to the 

 wall of the cranium. 



The tuberculum olfactorium is very large and strongly convex 

 ventrally. The medial borders of the two tubercula are divaricated 

 posteriorly, exposing a portion of the anterior perforated space in front 

 of the optic chiasma which probably includes the diagonal band of 

 Broca (PL XXII). 



The lateral olfactory tract arises from the ventrolateral border of 

 the olfactory bulb and can readily be seen as a clear white stripe accom- 

 panying the fissura endorhinalis. This band of fibers lies distinctly on 

 the ventromedial side of the fissura, that is, within the tuberculum 

 olfactorium, though we may infer by analogy with other mammals that 

 many of the fibers are distributed within the pyriform lobe on the other 

 side of the fissure. Microscopic examination will probably show that a 

 portion of the tract lies in the floor and walls of the fissure. 



The tuberculum olfactorium in mammals generally is a basal, that 

 is, subcortical, reflex center, receiving olfactory fibers of the second 

 order from the olfactory bulb. The pyriform lobe, on the other hand, 

 is a structure of transitional type. So far as it and the underlying 

 amygdala receive secondary olfactory fibers from the lateral olfactory 

 tract it should be considered as a part of the basal secondary olfactory 

 area (nucleus olf actorius lateralis) . So far as its differentiated cortex is 

 in physiological connection with this secondary olfactory area it should 

 be regarded as archipaliial in type, i. e., olfactory cortex of the same type 

 as the hippocampal cortex. So far as its cortex is in physiological con- 

 nection with the non-olfactory thalamic projection fibers it is neopallial, 

 i. e., of the same type as the dorsal cortex. The surface characteristics 

 suggest that in Ccenolestes these three components of the pyriform lobe 

 are very incompletely differentiated, thus resembling the still more 

 generalized reptilian condition (cf. Elliot Smith, '10, and Crosby, '17). 

 In higher mammals, on the other hand, these components of the pyriform 

 lobe attain much more distinct spatial localization in the gyrus hippo- 

 campi formation, though even in the human brain we find the same 

 transitional type of structure in the uncus region. 



The cerebellum. The cerebellum is smaller and simpler than in any 

 hitherto described mammals with the exception of Notoryctes and 



