NERVOUS SYSTEM. VERTEBRATA. 79 



although to all appearance nearly solid it will be seen to 

 be formed o a single sheet of nervous tissue thrown into a 

 complex series of transverse folds, between each of which 

 penetrates a part of the general cavity of the organ. This 

 complex folding is manifestly only a further extension of 

 the simple transverse grooves seen in Acanthias or Galeus. 

 The position of the original median transverse furrow is 

 marked by the deepest and most complex fold. 



The lobi inferiores are small and open widely into the 

 infundibulum. 



The medulla is considerably shortened antero-posteriorly. 



0. C. 1311 B ft. 



D. 77. The cranium of a Tope (Galeus communis) showing the 

 brain in situ. The brain affords in its general construction 

 and proportions an excellent example of that of an Elasmo- 

 branch, in which all the typical features are strongly marked 

 without any excessive specialisation of any one part. Thus 

 it is long and narrow, though not so much so as in low types 

 like Notidanus ; the optic and more especially the olfactory 

 centres are highly developed ; the cerebellum is large and 

 transversely fur rowed, though not so complex as, for instance, 

 in Lamna\ the medulla is of some length, though far shorter 

 than in primitive forms. Beyond these more general 

 features, in which this brain occupies a central position 

 among those of Elasmobranchs, it should be noticed that 

 the olfactory bulbs, although not double as in Carcharias, 

 are very distinctly bilobed. They are united to the cere- 

 brum by short, thick, hollow peduncles. 



The cerebrum is of the massive type with strongly 

 thickened lamina terminalis, and has upon its dorsal surface 

 two pairs of rounded eminences in place of the single pair 

 noted in Acanthias ; it shows no other external sign of 

 lateral separation. The cerebellum has four main transverse 

 fissures besides a few somewhat more partial and indistinct. 

 The lobi inferiores, hypophysis, and saccus vasculosus are 

 prominent, and very typical in form and arrangement. 



D. 78. The brain of a Tope (Galeus communis) isolated. The 

 vascular roof has been removed from the rhomboid fossa. 



0. C. 1311. Hunterian. 



