46 A NEW CRANIAL NERVE IN SELACHIANS. 



3. In Raja — I have observed this nerve in three species of the genus Raja. 

 Its point of connection with the brain is on the anterior dorsal surface instead of the 

 ventral surface as in Mustelus and other forms to be described later. From its point 

 of attachment to the brain-wall (PI. V, Fig. 4, n. nov.) it passes over the surface of 

 the forebrain, along the inner margin of the slender tractus, and upon the olfactory 

 cup. Here it runs along the anterior border of the fila olfactoria and dips among 

 the fibres of the latter about midway between the posterior end and the anterior 

 tip of the olfactory cup. In the skate there is no very obvious separation of the fila 

 olfactoria into two divisions as in the other forms described. There is, however, a 

 median blood-vessel that partly separates the fila into two divisions which corre- 

 spond, I think, to the lateral and median divisions already described. I have not 

 traced the nerve-fibres in the skate into connection with the nasal membrane. 



4. In Carcharias littoralis — Of all the selachians I have examined, the new 

 nerve is most readily seen in the sand-shark, Carcharias littoralis. It is always more 

 or less difficult to see this nerve before the brain has been stained. I have uni- 

 formly immersed the entire brain in borax carmine from ten to twenty-four hours 

 in order to secure a surface stain before completing its dissection in alcohol under a 

 dissecting microscope. In Carcharias (PL V, Fig. 5) the nerve can readily be de- 

 tected before staining. It is connected with the ventral surface of the brain (Fig. 6) 

 as in Mustelus, considerably in front of the optic nerve. The course of the nerve 

 from this ventral connection is nearly in a straight line to the olfactory cup. It 

 runs obliquely towards the long slender tractus and, after reaching it, runs for a part 

 of its course directly upon the tractus. The result is that the nerve enters the fissure 

 between the bundles of the fila olfactoria from behind, instead of crossing the median 

 division (n. olf. m) as in Squalus and Mustelus. 



In the Carcharias there is a bundle of the fila olfactoria connecting the two divi- 

 sions of the olfactory nerve. The new nerve as it enters the fissure branches une- 

 qually, sending a small twig to the connecting bundle, and a main stem to the lateral 

 division. This main stem dips into the mass of fila making up the lateral division 

 and, after coming into contact with the enveloping membrane of the capsule, con- 

 tinues its course along the latter in an antero-lateral direction. From time to time 

 it gives off small branches, which penetrate the investing membrane of the capsule 

 and enter the folds of the nasal epithelium. The chief part of the nerve reaches the 

 antero-lateral portions of the cup and there disappears within the nasal membrane. 

 As in other cases, it preserves throughout its course a striking independence. 



The nerve has a small ganglionic enlargement near the base of the bulb. 



5. In Sphyrna — A part of the brain of the bonnet-head, Sphyrna tiburo, 



