A NEW CRANIAL NERVE IN SELACHIANS. 47 



is represented in Figure 7 (PL V). The forebrain is expanded laterally, but is short 

 antero-posteriorly. The olfactory cups are elongated and shaped somewhat like a 

 long seed-pod. The tractus is relatively slender. The new nerve unites with the 

 brain on the ventral surface (Fig. 8, n. nov.) in a depression much nearer the optic 

 nerve than in the other forms described. From this point the nerve passes directly 

 to the base of the fila olfactoria and mingles with them in a manner I was unable 

 to make out clearly in the single specimen of this shark at my command. 



There are very well marked ganglia (Fig. 8, gn, gn') on the nerves. In the speci- 

 men observed the ganglia were not symmetrical in position. The one on the left side 

 could be seen from above just in front of the brain (Fig. 7, gn'). The one on the 

 opposite side was on the base of the brain, and could be seen only in ventral view 

 (Fig. 8, gn). 



6. In Scoliodon terras-novae —In a shark from Florida, identified by Mr. A. D. 

 Howard as probably Scoliodon terrse-novse, the new nerve (PL V, Fig. 9, n. nov.) 

 is likewise present. In this shark the separation of the great divisions (n. olf. I., 

 n. olf. m.) of the fila olfactoria is very striking. Each division is of course made up 

 of a number of bundles of fila. 



The nerve enters the brain substance on the ventral surface, in the median plane 

 about midway between the optic nerve and the anterior tip of the forebrain. It is 

 flattened on the ventral surface of the brain, and has an enlargement which is possibly 

 ganglionic. Beyond this swelling the nerve is slender and passes directly to the 

 lateral division of the fila olfactoria, with which it unites. It does not branch in the 

 fissure between the two great divisions as described in several other forms. Near 

 the base of the median division there is a slight enlargement which is also possibly 

 ganglionic. 



7. In Other Selachians — I have also determined the existence of this nerve in 

 embryos of the torpedo, and, as mentioned above, in three species of Raja. This 

 makes a total of nine species of selachians in which this nerve has been found. It 

 will be interesting to determine whether it is present or absent in others.* 



The anatomical descriptions show that there is considerable variation as to the 

 position on the external surface of the brain at which this nerve enters. In the 

 skate it enters on the anterior dorsal surface; in Squalus it enters the brain in the 

 median furrow, midway between the dorsal and ventral surfaces; and in the other 

 forms examined it enters on the ventral surface, rather posteriorly in Sphyrna, and 

 further anteriorly in the others. As to internal connections there is doubtless 



* Since the above was sent to the printer I have identified this new nerve in 19 genera and 24 species of adult 

 selachians. 



