242 Journal ok Comparative Neurology. 



bellar ventricle is somewhat larger; the remainder of the 

 cerebellum is precisely as in the carp. 



In the older individuals the great lateral expansion of the 

 volvula is remarkable. 



The extreme in this type of cerebellar differentiation is 

 reached in the black horse, Cvcleptus elongatus. In this case' 

 the optic lobes are thrust far cephalad over the cerebrum and 

 expanded laterally like a hood. The tectum is v^^edged apart 

 and the two tori are connected by a wide stretch of thin 

 membrane only. The volvula is enormous, and consists of a 

 central and two lateral lobes, in which the simple fold, which 

 is the real source of the structure, is effectually obscured. 

 The lateral lobes are pushed cephalad as divergen tuber- 

 osities. The appearance is as though these bodies had been 

 formed by excessive lateral growth, which, among other 

 things, had removed the granular layer entirely from the 

 dorsal lamina and then crumpled the whole organ by lateral 

 pressure. The middle lobe is morphologically the ventral 

 lamina, which is thrust between the the two moities of the 

 dorsal lamina. 



This explanation solves the apparently unreconcilable 

 conditions. The lateral lobes or severed portions of the 

 dorsal lamina of the volvula extend far caudad of the point 

 where the ventral lamina emerges as the roof of the fourth 

 ventricle or vermiform portion of the cerebellum. 



The presence of these two latero-caudal excrescences on 

 either side of the median portion of the cerebellum is the 

 obvious external cause of a median infolding of its dorsal 

 roof, which is indicated even in the external view by the 

 groove seen in Fig. 4, Plate XIX. In transverse sections the 

 fold is seen to be an exceedingly deep one, though the edges 

 coalesce completely. 



The remainder of the cerebellum is as in the carp, except 

 that the mode of connection with the medulla is modified by 

 the lateral pockets containing the divaricated caudal protru- 

 sions of the volvula. 



