55 



ganglionic enlargement (distal ganglion), in front of which two branches 

 are given off. It then crosses obliquely the bulbus and the median di- 

 vision of the olfactory nerve {n. olf. m.)^ and enters the fissure between 

 the two great divisions of the fila olfactoria. Just as it reaches this 

 fissure it branches unequally, the main stem taking a course similar 

 to the corresponding branch io Squalus. The fibers of the chief 

 branch are distributed mainly to the antero-lateral portion of the ol- 

 factory cup. 



InMustelus laevis. ^ — The brain is similar to that of Mustelus 

 canis. The olfactory cups are relatively smaller, and the tractus is 

 short and stout, so that, in some specimens, the frontal margin of 

 the prosencephalon comes in contact with the median division of the 

 bulbus. 



The central connection of the nerve is on the base of the pros- 

 encephalon as in Mustelus canis. There is a long filiform ganglion 

 on the nerve near the base of the bulbus. In front of the ganglion 

 the nerve branches. It is very slender in front, and somewhat thicker 

 behind, so that the posterior limit of the ganghon is difficult to de- 

 termine by surface examination. 



As in the other forms described, the nerve enters the fissure se- 

 parating the two divisions of the olfactory nerve and connects mainly 

 with the outer division. 



InGaleus canis. — In several specimens ofGaleus canis obtained 

 at the Naples Station, the general resemblance of the brain to that of 

 Mustelus canis ^), of the west Atlantic coast, is evident, but there are 

 differences as to the new nerve. In Mustelus canis, of which I have 

 dissected many specimens, the ganglion is inconspicuous, but, in Galeus 

 canis, of which I have had three specimens, there is a very well 

 marked ganglion. It is, in fact, the largest ganglion I have found on 

 this nerve in any selachian. 



Fig. 20 shows a sketch of the brain of Galeus canis as seen from 

 above. A comparison of this with Fig. 19, shows that the olfactory 

 cups are relatively larger, and the olfactory tractuss lightly shorter in 

 G. canis, but that the thalamencephalon is considerable longer in M. 

 canis. It should be stated, however, that there are individual variations 

 in the brains of the same species, as one who compares a large number 

 of the brains will surely notice. 



1) Untersuchuügeri über den feineren Bau des Fischgehirns, 1878, 

 Taf. I, Fig. 6. 



