53 



followed further. Its close union with the tractus reminds one of the 

 similar condition of this nerve in Amia. 



In Lseviraja oxyrhynchus. — The brain of Laeviraja (Fig. 18) 

 resembles that of Raja. The new nerve is dorsal in position, but its 

 point of attachment with the brain 

 is more lateral than in Raja. As 

 shown in Fig. 18B, it bears a 

 small rounded ganglion in the 

 space between the bulbus and the 

 prosencephalon. 



n. nov. 



poc. olf. 



] 



B 



Fig. 18. Dorsal \\ew of the brain of 

 an andult Lseviraja oxyrhynchus, nat. size. 

 B, slightly enlai'ged view of a ijortion of the 

 right side of the same. 



B. Forms in which the 

 Nerve has a ventral Con- 

 nection with the Brain- 

 wall in the Adult. 



The dorsal attachment of this 

 nerve to the brain in the eleven 

 genera described above is of 

 general interest. It sets aside the 

 idea that this nerve is usually, or 

 normally, connected with the ven- 

 tral surface, as has been sug- 

 gested, on account of its position 

 in Amia, Ceratodus and Pro- 



topterus. There is to follow a description of the nerve in nine genera 

 in which its attachment is with the ventral surface of the brain, but 

 there is reason to believe, that in some cases, at least, this is a se- 

 condary rather than a primary position. In observations on embryos 

 of Mustelus canis, I have determined that it starts on the dorsal sur- 

 face, and becomes carried to its ventral position through the unequal 

 growth of the parts of the brain. 



In Musteluscanis. — The brain of the smooth hound of the west 

 Atlantic coast (Mustelus canis) is represented in Fig. 19, as seen from 

 above. The olfactory cups are relatively large, suggesting that the 

 olfactory apparatus is, doubtless, of great importance in the life of 

 this animal. The olfactory tractus is relatively shorter than in Squalus 

 acanthias, and the prosencephalon is not divided as in that form. 



The new nerve is more easily detected in Mustelus than in Squalus 

 for it lies more freely in the membrane of the cranial cavity and is 

 not so closely applied to the tractus. Its point of connection with 

 the brain-wall is on the ventral surface as shown in the lower part of 



