70 PHYSIOLOGICAL SERIES. 



8QUAUDA. 



D. 68. The brain of Notidanus griseus exposed within the skull 

 from the dorsal and ventral aspects. 



Tin- Klasmobranch brain is found in its simplest and most 

 primitive condition in the Notidanidse, and presents in 

 them many features that in other members of the class are 

 transient, occurring only during certain developmental 

 stages. The following characters should be particularly 

 noticed as indications of primitive construction : The rela- 

 tively great length and narrowness of the brain ; the absence 

 of any marked local thickening of its walls ; their general 

 thinness and the consequent spaciousness of the ventricles 

 (for these and other internal features see Maclay, t. ii. 

 Hg. 12) ; the simple unconvoluted cerebellum ; the great 

 development of the medulla, its length, wide dorsal open- 

 ing, and gradual passage into the cord. 



The fore-brain (cerebrum) is deeply cleft anteriorly 

 (more so than in any other Elasmobranch) by an infolding 

 of the lamina terminalis ; its cavity is thus separable into 

 a posterior unpaired chamber the fore part of the third 

 ventricle passing in front into a pair of anteriorly directed 

 pockets, or lateral ventricles. The latter are continuous at 

 their anterior end with the cavities of the long olfactory 

 j)eduncles, by means of which the olfactory bulbs, which are 

 situated directly beneath the olfactory organ, are connected 

 to the rest of the brain. The olfactory bulb has always 

 in Elasmohranchs (except it seems in Echinorhinus) this 

 close relation to the olfactory organ, so that the length 

 n\ the peduncle varies in different forms according to 

 the portion of the organ with regard to the brain. The 

 pedunelr i> usually, as in this case, hollow, but sometimes 

 when very long it is solid ; it has the same structure as the 

 olfactory bulb, and together with it is an outgrowth from 

 the fore-brain. 



Tin- thalainenee|.haloM is comparatively short and wide. 

 The tore part of its roof is membranous and forms a conical 

 sac-like protrusion (paraphysis) betv/een the hemispheres. 

 The hinder part i- eneeale,| by the optic lobes: it contains 

 the ganglia haheiiulaj and j;ive.- origin to the thread-like 



