DEVELOPMENT OF ELASMOBRANCH FISHES. 213 



Gegenbaur, form five separate nerves, each indicating a seg- 

 ment. A comparison of the post-auditory nerves of Scj^Uium 

 and other typical Elasmobranchs with those of Hexanchus and 

 Heptanchus proves, however, that other segments w^ere origin- 

 ally present behind those now found in the more typical forms. 

 And the presence in Scyllium of numerous (twelve) strands 

 from the brain to form the vagus, as well as the fact that a 

 large section of the commissure connecting the vagus roots wdth 

 the posterior roots of the spinal nerves is not connected with 

 the brain, appear to me to shew that all traces of the lost nerves 

 have not yet vanished. 



Passing forwards from the post-auditory nerves, we come to 

 the seventh and auditory nerves. Th« embryological evidence 

 brought forward in this paper is against regarding these nerves 

 as representing two segments. Although it must be granted 

 that my evidence is not conclusive against an independent 

 formation of these two nerves, yet it certainly tells in favour of 



their orio^inatinoj from a common rudiment, and Marshall's re- 

 ts o ' 



suits on the origin of the two nerves in Birds (published in 

 the Journal of Anatomy and Physiology ^ Vol. XT. Part 3) sup- 

 port, I have reason to believe, the same conclusion. Even 

 were it eventually to be proved that the auditory nerve origin- 

 ated independently of the seventh, the general relations of 

 this nerve, embryological and otherwise, are such that, pro- 

 visionally at least, it could not be regarded as belonging to 

 the same category as the facial or glossopharyngeal nerves, and 

 it has therefore no place in a discussion on the segmentation 

 of the head. 



The seventh norve of the embryo (PL xvi. fig. 1, vii.) is 

 formed by the junction of three conspicuous branches, (1) an 

 anterior dorsal branch which takes a more or less horizontal 

 course above the eye (vii. a) ; (2) a main branch to the hyoid 

 arch (vii. hy); (3) a smaller branch to the posterior edge of the 

 mandibular arch (vii. mn). The first of these branches can 

 clearly be nothing else but the typical "ramus dorsalis," of 

 which however the auditory may perhaps be a specialized 

 part. The fact that this branch pursues an anterior and not 

 a directly dorsal course is probably to be explained as a con- 

 sequence of the cranial flexure. The two other branches of 



