NERVOUS SYSTEM. VERTEBRATA. 81 



relatively great development of the medulla, its gradual 

 passage into the cord, and widely open rhomboid fossa. In 

 contrast to the large medulla, the cerebellum is small, its 

 greater part consisting (as in the Eays) of the posterior lobe. 

 Tts sides are somewhat swollen to form a pair of lateral lobes 

 (peduncles) close in front of the medullary auricles. The 

 optic lobes are oval and of moderate size ; the lobi inferiores 

 scarcely distinguishable ; the hypophysis small ; and the 

 saccus vasculosus and infundibuluin very large. It should 

 be particularly noticed that the brain combines features 

 characteristic respectively of the Sharks and Rays the 

 cerebrum and medulla conforming to the former type, the 

 cerebellum to the latter. The brain occupies only a small 

 part of the spacious cranial cavity. 



RAJIDA. 



D. 82. The cranium of a Skate (Raja batis) with the brain 

 exposed. The brain occupies only a small part of the 

 cranial cavity, and is chiefly remarkable for the strong 

 development of the olfactory centres and for the antero- 

 posterior shortening of the medulla and the prominence 

 of the auricles and lobi linesQ lateralis. 



The olfactory bulbs are solid and differ considerably in 

 shape from those of Sharks ; each is laterally lengthened 

 and bears a somewhat similar relation to its peduncle that 

 the foot of a stocking does to the leg, the heel being repre- 

 sented by the swelling at the end of the peduncle, and the 

 foot by the part of the bulb that extends outwards along the 

 posterior surface of the olfactory capsule. The peduncles 

 are solid and very long ; each terminates in a strongly pro- 

 nounced swelling on the lateral surface of the cerebrum. 

 Owing to the presence of these the cerebrum is remarkably 

 broad ; it is convex anteriorly, but has no median groove; 

 upon its dorsal surface are a pair of indistinct eminences. 

 The thalamencephalon and mid-brain are well developed 

 but in no way remarkable. The former is somewhat 

 longer and narrower than in many Rays. The optic lobes, 

 although dwarfed by the great size of the fore-brain, are 

 in reality well-developed ; the lobi inferiores, infundibuluin, 

 VOL. II. G 



