48 



poc. olf. 



n. nov. 



shorter than in Heptanchus but the olfactory tracti are more elongated. 

 The ventral portion of the prosencephalon (pars olfactoria) extends 

 anteriorly beyond the dorsal elevation, in the form of two enlargements 

 separated by the median furrow. The olfactory cups are large pig- 

 mented, (as is usually the case in the selachians), and bilobed. The 

 bulbus at the base of the olfactory cup is large, and is followed by 

 the long slender olfactory tractus. In the specimen sketched, the con- 

 nective tissue envelope was trimmed so closely at the base of the cup, 

 that the larger bundles of the olfactory nerve were shown. In most 

 of the other sketches these are not shown. 



The new nerve in Hexanchus is relatively large and easily ex- 

 posed. It penetrates the brain -wall by a very slender root, deep 

 within the median furrow of the prosencephalon. From this point, it 

 passes forward almost in a direct line to the olfactory cup. Its central 



portion is slender but, as it crosses the 

 bulbus, it is conspicuously widened. 

 Whether or not this enlargement is 

 partly ganglionic has not been de- 

 termined. At the base of the cup the 

 nerve branches unequally and passes 

 into the fissure separating the two di- 

 visions of the olfactory nerve. The 

 main branch of the nerve in Hexanchus, 

 as far as I have been able to determine, 

 joins with the median rather than the 

 lateral division of the olfactory nerve. 

 In Centrophorus granulo- 

 sus. — The brain of this form 

 (Fig. 13) is very similar to that of Squa- 

 lus acanthias as might be expected from 

 their close relationship. The olfactory 

 cups are relatively smaller than in Squa- 

 lus, and instead of being rounded have 

 more of a bellshaped outline. The bul- 

 bus is relatively large. The new nerve 

 penetrates the brain-wall by two roots within the median furrow. The 

 points at which the roots enter are nearer the dorsal than the ventral 

 surface. The filiform ganglion (gn) is in the space between the front 

 of the prosencephalon and the olfactory tractus. In front of the 

 ganglion the nerve passes onto the tractus, and traverses the bulbus 

 in the median plane, not crossing it obliquely as in Squalus, Mustelus 



n. opt. 



Fig. 13. Dorsal view of the brain 

 of Centrophorus granulosus, nat. size. 



