59 



There is a very well marked lozenge- shaped ganglion {gn, gn') on 

 each nerve. In the specimen observed the ganglia were not symme- 

 trical in position; the one on the left side could be seen from above, 

 just in front of the brain {gn'), while the one on the opposite side was 

 on the base of the brain and could be seen only in lateral view {gn). 

 In addition to these prominent ganglia there is a minute distal one 

 near the olfactory cup. It is in front of this distal ganglion that the 

 nerve forks as mentioned above. 



In Sphyrna zygsena. — In the hammer- headed shark, there 

 is a marked condensation of the parts of the brain in an antero- 

 posterior direction. The fore -brain is large and rounded, and the 

 complex cerebellum completely covers the mid-brain extending, by a 

 frontal lobe, upon the thalamencephalon. Owing to the great lateral 

 expansion of the head, the olfactory tractus is elongated; the olfactory 

 cup is similar in form to that of the bonnet -head, but is relatively 

 longer and more slender. 



The anatomical relations of the new nerves are similar to those in 

 S. tiburo. It starts from a corresponding position on the ventral sur- 

 face of the brain, and bears on each side, a distinct lozenge -shaped 

 ganglion. The new nerve was traced to a distance of 14 mm from 

 its brain attachment, but was thereafter broken and lost in the dense 

 connective tissue near the base of the bulbus. 



In Alopias vulpes. — The general outline of the brain is 

 similar to that of Myliobatis, but with longer olfactory cups and rela- 

 tively shorter tractus. The cerebellum is very large and complicated, 

 its top is convex and rises high above the crest of the fore-brain. It 

 is in contact with thelamencephalon and overhangs the lobes of the 

 mid-brain. 



The new nerve is very readily seen. Its central connection is on 

 the ventral surface, within the median furrow (there is no furrow on 

 the dorsal surface), about midway between the optic chiasma and the 

 frontal margin of the prosencephalon. The nerves are not, as in so 

 many forms, exposed upon the ventral surface, but are enclosed within 

 the median furrow, from which they must be removed before they 

 become evident. 



An elongated ganglion is found on the nerve near the base of the 

 bulbus. In microscopic structure this ganglion is notable, on account 

 of the very large nerve- cells distributed through it (see Fig. 2 B). 



In front of the ganglion, the nerve branches, unequally, sending 

 a slender branch to the median division, and a larger one to the 

 lateral division of the olfactory nerve. 



