174 JouHNAi. OK Comparative Neurology. 



The valve thus becomes restricted and the cephalic lobe of 

 the cerebellum is rolled upon itself (Plate XI, Figs. 7, 8). 

 One result of this modification is the great distension of the 

 optic ventricle and commensurate thinning of the tectum pos- 

 teriorly. The colliculi are also reduced in size thereby. 



In Lepidosteus the adhesion of the valve dorsally upon a 

 forw^ard tongue of cerebellum ventrad is considerable, but in 

 Accipenser the ventral (cerebellar) portion is pushed for- 

 ward and rolls cephalo-dorsad, fairly filling the cavity. The 

 structure of this body, which may be called volvula ccrebelli 

 or volvula, is identical with that of the cerebellum proper. 

 The Cerebelhim. — In the development of the cerebellum 

 we have the most characteristic feature in the brain of the 

 ganoid fishes and, at the same time, the peculiar modifi- 

 cations of the theme express the generic variations most dis- 

 tinctly. In the gars, where the whole brain is quite reptilian, 

 the cerebellum is highly developed superficially, while in the 

 sturgeon only a relatively small part is exposed, the 

 remainder being packed in the cavity of the mesencephalon 

 and the laterally enlarged fourth ventricle. In Lepidosteus 

 the cerebellum may be divided into the following parts: 

 first, a cephalad invasion of the mesencephalic cavity, more 

 or less closely associated with the valve, constituting the vol- 

 vula as above described; second, the median lobe or vermi- 

 form lobe, third, two lateral lobes\ fourth, a posterior pouch 

 or bursa. 



As a whole, and excepting the volvula, the cerebellum 

 resembles that of the alligator. The vermiform process is 

 moderately convoluted, so that in longitudinal section there 

 is anteriorly a caudad invagination, then a cephalad projec- 

 tion carrying a part of the ventricle forward as an anterior 

 recess lying dorsad to the pocket formed from without, due 

 to the invagination or concrescence above-mentioned. The 

 main cavity of the vermiform lobe is pentagonal in transec- 

 tion, giving off diverticles into the cavities of the lateral 

 lobes. 



