464 'Journal of Comparative Neurology and Psychology. 



end of the brain, and the discussions of v. Kupffer, Koltzoff and 

 others do not seem to the writer to offer anything of value in the 

 present state of our knowledge. (See note, p. 535.) 



It should be noted that the ridge in the brain floor which contains 

 the optic chiasma* and other decussating tracts is especially high and 

 prominent in cyclostomes and that the pit behind the ridge, recessus 

 postopticus, is quite as deep and sharply marked as that in front of 

 the ridge, recessus prceopticus (commonly called recessus opticus). 



2. Selachians. 

 Kotes on Head Morphology. 

 In the adult selachian brain the diencephalon presents no peculiar- 

 ities of especial importance to our problems. The limit between the 



Nucleus habeaulae^ Tectum mesencephaU 

 Velum transversum 



Epiphysis i 



Cerebellum 



Paraphysis 

 Telencephalon 



Com. infima 

 Lobus visceralis 



Rec. neurop. 

 Rec. praeop. 



Lobus inferior 



Optic chiasma 



Fig. 1. The mesial surface of the right half of the brain of Mustelus. 

 From Johnston, 1906. For the significance of the abbreviations used on all of 

 the figures, see the list at the end of this article. 



diencephalon and telencephalon is clearly marked dorsally by a deep 

 velum transversum which forms the anterior wall of the dorsal sac 

 belonging to the diencephalon, and the posterior wall of the para- 

 physis belonging to the telencephalon (Fig. 1). In front of the para- 

 physis the membraneous roof is complexly infolded to form the plexus 



