NERVOUS SYSTEM. VERTEBRATA. 75 



with those previously described, shows a noticeable increase 

 in the size of the parts in connection with the olfactory 

 organs (olfactory bulbs, peduncles, and cerebrum). On the 

 other hand, the optic region is weak. The olfactory bulbs 

 are each partially divided into a right and left half by a 

 shallow groove an indication of incipient duplicity, of 

 interest in view of the complete separation of the olfactory 

 bulbs into two parts in Carcharias. They are united by 

 short thick peduncles to the lateral parts of the cerebrum. 

 The latter is strongly convex in front and shows but slight 

 indications externally of separation into lateral lobes ; its 

 postero-dorsal parts are raised to form a pair of rounded 

 eminences similar to those seen in Acanthias. The cerebrum 

 is continuous behind with the remarkably long and narrow 

 thalamencephalon. The tela choroidea forms a close-fitting 

 membranous roof to this region, and shows very clearly 

 between its posterior fourths an opaque band the com- 

 niissura habenulte. The epiphysis, which rises close behind 

 this commissure, is long and thread-like as in most Elasmo- 

 branchs ; it extends forwards over the cerebrum and 

 terminates on a level with the olfactory bulbs in a thickened 

 end. In this and most other genera it is said to be solid 

 except close to the brain, and to consist of cells embedded 

 in a finely granular intercellular substance. 



The cerebellum is quite simple, though of some size ; it 

 has a median longitudinal furrow and slight lateral inden- 

 tations. (The asymmetry noticeable in this cerebellum is 

 probably unnatural, due to distortion during hardening.) 

 The medulla is of a higher type than in the previous speci- 

 mens ; it is shorter and broader and has a more definite 

 limit towards the cord. The brain fills the greater part of 

 the cranial cavity. 



D. 73. The brain of a Spotted Dog-fish (Scyllium catulus) opened 

 to show its internal structure (fig. 20) . The roof of the cere- 

 brum has been removed to show the direct continuity of the 

 unpaired part of the cerebral cavity with the third ventricle 

 behind, and with the lateral ventricles in front. The latter 

 are separated by a thick median septum, and are continued 

 from their postero- lateral parts through the olfactory 



