MUSTELUS L/EVrS. 103 



up into several brandies innervates the organs of the com- 

 missure. The nerve is thus pushed forward, out of a direct 

 course, by the trunk muscles, as they push forward upon 

 the skull. 



Posterior to the commissure the lateral canal of the body 

 begius, but the innervation of its sensory organs could not 

 be determined in either of my series of sections, the nerves 

 that innervate them traversing the superficial layers of the 

 adjacent muscles, and being lost owing to sbght breaks in 

 the sections. A certain number of the most anterior organs 

 seemed to be innervated by a dorsal branch of the glosso- 

 pharyngeus. This branch was an important one, and branches 

 from the lateral canal organs ran toward the point where it 

 was bi'oken, and lost, as it passes upward over the dorso- 

 lateral corner of the skull. Another nerve, which arises 

 as a branch of the supratemporal branch of the nervus 

 lineee lateralis, close to its base, is also broken and lost 

 in the muscle-fibres here, and it might be it and not the dorsal 

 branch of the glossopharyngeus that innervates the anterior 

 organs of the lateral canal. 



If certain of the anterior organs of the lateral canal are 

 innervated by the dorsal bi-anch of the glossopharyngeus, 

 as seems probable, it is to be noted that they certainly He 

 posterior to the supratemporal commissure. In i\mia the 

 glossopharyngeal organs lie anterior to the commissure, 

 and Ewart assumes (18) that organs so innervated are pi^o- 

 bably found in a similar position in most elasmobranchs. 

 As Ewart definitely finds, in Lgemargus, a section of the main 

 infra-orbital canal that lies immediately anterior to the com- 

 missure innervated by branches of the nervus lineiB lateralis, 

 it is evident that the commissure of L^margus and that of 

 Mustelus do not arise from similar points of the main line. 

 If the commissure can thus, in principle, shift forward and 

 backward in its point oF origin from the main canal, it is 

 evident that it must be used with some reserve in seeking 

 to establish the homologies of the related bones in the skulls 

 of teleosts and ganoids. 



VOL. 45, PART 2. NEW SERIES. J 



