NERVOUS SYSTEM. VERTEBRATA. 77 



Close behind the point of origin of the epiphysis, the cut 

 edge of the roof of the brain is thickened by the presence 

 of the posterior commissure. The mid-brain contains a 

 considerable cavity (aqueduct of Sylvius) which sends 

 lateral offshoots into the optic lobes. The roof (tectum 

 opticum) of this region is moderately thick ; its division 

 into outer and inner layers is very evident upon the section 

 surface. 



In dividing the cerebellum the roof has been cut along 

 its right lateral margin and turned aside entire with the 

 left half of the brain. The thin simple character of its 

 walls, and the complete course of the longitudinal zones from 

 the medullary auricles to the boundary line between the 

 cerebellum and mesencephalon are shown. 



D. 74. Part of the skull of a Basking Shark (Selache maxima) 

 with the brain exposed from above. The brain is relatively 

 small and somewhat shorter and broader than those pre- 

 viously described ; it occupies a small part only of the 

 spacious cranial cavity. The cerebrum is globular with 

 thickened (?) walls, and is superficially without signs of 

 lateral division. The olfactory bulbs and peduncles have been 

 destroyed, but the rootof the (apparently solid) right peduncle 

 can be seen rising from a slight eminence at the antero-lateral 

 corner of the cerebrum. The thalamencephalon is very 

 short, and is completely hidden in dorsal view between the 

 cerebrum and optic lobes. The latter are of considerable 

 size and are covered by the anterior lobe of the cerebellum. 

 The cerebellum is large, with a series of twelve or so deep 

 transverse fissures. 



The fish from which this dissection was made was caught 

 in the North Sea ; it measured 12 feet in length. 



0. C. 1311 A b. Presented by Dr. Albert Gunther. 



D. 75. A similar dissection of the hinder part of the skull of a 

 somewhat larger Basking Shark (Selache maxima). The 

 parts of the brain anterior to the optic lobes are absent. 

 In this specimen the large size and convoluted structure of 

 the cerebellum are very clearly shown ; it should be noticed 

 that the median transverse furrow is more pronounced than 



