60 





In Scyllium stellare. -In this form, the olfactory cups and ol- 

 factory bulbi are very large, as compared with the size of the brain. 



The sketch (Fig. 23), 

 /f?-^'"^-^-^,^ was made from the 



brain of an adult 

 specimen 78 cm long. 

 The new nerve is 

 relatively easy to dis- 

 sect. Centrally, it is 

 connected with the 

 brain, on the ventral 

 surface, a little in front 

 of the middle point 

 between optic chiasma 

 and anterior margin of 

 the prosencephalon. 



The lozenge-shaped 

 ganglion (gn) , lying 

 close to the bulbus, 

 is readily seen from 

 above. Microscopic ex- 

 amination of sections 

 shows that the ganglion 

 cells are larger than in Squalus (Fig. 2 A), but smaller than those 

 of Alopias (Fig. 2 B). 



In front of the ganglion, the two principal branches of the nerve 

 pass obhquely across the bulbus, as shown in the figure, to the fissure 

 of the olfactory nerve. One branch joins the median, and the other 

 the lateral division of the fila olfactoria. The chief stem subdivides 

 and some of its smaller branches have been traced through the con- 

 nective tissue covering of the olfactory cup into its interior. 



The new nerve has also been observed in Scyllium canicula and 

 found to be very similar to that of S. stellare. 



In Pristiurus melanostomus. — The brain of Pristiurus is 

 represented, natural size, in Fig. 24 A. At B, the same figure, the 

 olfactory cup is shown with a ganglion at the base of the bulbus ; at C, an 

 outline diagram of the fore part of the brain as seen from below; 

 and, at D, the ganglion with its two roots. 



The new nerve is connected with the ventral surface of the brain 

 by two roots, as shown at C and D, Fig. 24. The roots are both 

 slender and rather widely separated. The median one is larger than 



Fig. 23. Dorsal view of the braiuof an adnlt 

 Scyllium stellare, nat. size. On the right the ganglion 

 slightly enlarged. 



