NEKVOUS SYSTEM. VERTEBRATA. 71 



epiphysis. The floor is evaginated in the mid-line to form 

 the infundibulum. This is hidden in the specimen by the 

 hypophysis an ectodermic glandular structure closely 

 attached to its ventral surface. The lateral parts of a large 

 vascular dilatation of the posterior wall of the infundibulum 

 (saccus vasculosus) can be seen protruding beyond the 

 lateral parts of the hypophysis. On either side of the 

 infundibulum, in front of the saccus vasculosus, are a pair 

 of globular enlargements the lobi inferiores. Each of 

 them contains a cavity (not shown) in connection with that 

 of the infundibulum. 



The optic lobes are of considerable size, although neither 

 here nor in any other member of the group do they reach 

 the size and importance that they do in most Teleostean 

 fishes. They are oval in form, separated dorsally by a 

 superficial longitudinal groove, and contain a large com- 

 mon cavity. Their posterior part is covered by the anterior 

 lobe of the cerebellum. No part of the Elasmobranch 

 brain is subject to more variation than the cerebellum ; 

 in this species it occurs in its simplest form, although well 

 developed when compared with that of certain other 

 Vertebrate groups (e. g, Cyclostomes, Amphibia). It is 

 superficially perfectly smooth except for a slight median 

 longitudinal furrow, and when seen from above is diamond- 

 shaped with pointed anterior and posterior lobes, within 

 each of which extends a part of the common cavity. A 

 single median opening leads from the latter into the fourth 

 ventricle. On either side of the posterior cerebellar lobe 

 lies a convoluted portion of the thickened margin of the 

 rhomboid fossa ; this ridge (medullary auricle) is clearly 

 seen on the left side ; it passes to the mid-line under cover 

 of the posterior lobe of the cerebellum ; here meets with its 

 fellow of the opposite side, without fusion, and then, 

 turning abruptly upwards into the cavity of the cerebellum, 

 forms a projecting longitudinal ridge (dorsal longitudinal 

 zone of Burckhardt) that runs close beside the mid-line to 

 its anterior extremity. From their microscopic structure 

 the medullary auricles should be included in the cerebellum. 

 The course of this pair of zones is not visible in the 

 specimen after they pass under cover of the cerebellum. 



