MUSEUxM OF COMPARATIVE ZOOLOGY. 17 



The cerebellum is of medium size, rather smooth ou its upper surface, 

 rounded in front, and presents an acute angle — with blunted apex — 

 between the corpora restiformia. On the upper surface the longitudinal 

 depressions are partly due to the uneven floor of the ventricle, on which 

 the upper walls rest. There are three moderate transverse depressions. 

 In the cerebellum the amount of plication is greater than that in Hex- 

 anchus as figured by Maclay. There is some likelihood that his figure 

 is taken from a young specimen, and that a large one will be marked by 

 greater complication. In Maclay's figure of Hexanchus the folds are 

 represented by a simple upward line with a transverse bar on the top, 

 like a letter T. To represent the same section in the new shark, we 

 shall have to place another X on each end of the transverse bar. jVIaclay 

 figures a longitudinal section of the cerebellum of a young Mustelus, 

 which shows a pretty close agreement. An adult Mustelus, which is a 

 great deal more complex, is also figured. 



The corpora restiformia are comparatively large ; they approach each 

 other behind the cerebellum till there is but a small space between 

 them. 



The medulla is large, somewhat larger than the same portion in the 

 Notidanida). The waved appearance in the sinus rhomboidalis, fourth 

 ventricle, is caused by the transverse bands of fibres in its membranous 

 roof. 



The nerves of the third pair (^oculo-motorius) emerge from the lower 

 surface of the brain close behind the hypophysis, and a little farther 

 from the median line than the outer margins of the latter. 



Back of the optic lobes, beneath the cerebellum, are the roots of the 

 fourth pair (trochlearis) ; these nerves are very small. 



Not far from the middle line on the ventral surface, near the 

 anterior extremity of the medulla, are the roots of the sixth pair 

 (abducens) . 



In a group at the side of each of the corpora restiformia are four 

 roots, three of which go to make up the nerve of the fifth pair (trigemi- 

 nus) and the fourth root apparently forms the seventh and eighth 

 (facialis and auditorius). One of these roots emerges at the side, two 

 on the inferior surface, and one at the side of the fourth ventricle, 

 ■whence it passes outward by the side of the restiform bodies. 



The nerves of the ninth pair (glosso-jjharyngeus') emerge quite near 

 the roots of the vagus, between the first of the series and the sixth pair, 

 on the lower side. 



The tenth pair (vagus) is somewhat asymmetrical, having eight roots 



VOL. XII. — NO. I. 2 



