SENSORY CANALS OF L^MARGUS. 65 



is clearly indicated by its being innervated by the hyomandibular branch of the 

 facial. 



In addition to these three main cranial canals and the lateral canal of the trunk, 

 there are in Laemargus only two other canals : — (1) a canal (I.e., fig. 1, PI. I.) which 

 lies behind the auditory pores and serves to connect the canal systems 'of the two sides 

 (as this commissural canal is innervated by the lateralis nerve, it will be considered as 

 part of the lateral canal) ; (2) a short canal (p,L, fig. 1, PI. I.) continuous with the 

 lateral canal, which seems to form its most anterior portion, but as it is innervated by 

 the buccal branch of the facial nerve, it will be looked upon as belonging to the infra- 

 orbital canal. It thus appears that in Laemargus all the sensory canals are supplied by 

 two nerves — the vagus and facial, or to be more explicit, by the lateralis division of the 

 vagus and the ophthalmic, buccal, and hyomandibular divisions of the facial ; and hence 

 we might speak of the ophthalmic, buccal, hyomandibular, and lateral sensory canals. 



Hitherto in Selachians, the sensory (mucous) canals have been studied per se, with the 

 result that an extremely complicated nomenclature has arisen. For example, Garman (29), 

 who refers shortly to Laemargus under the name of Somniosus carcharias, indicates the 

 presence of fourteen cranial canals. In doing this he follows the system of Agassiz and 

 most other writers, and practically takes no heed of the relation of the canals to the 

 cranial nerves, as has been recently done to a considerable extent by Allis in his descrip- 

 tion of Amia. From the statement of Garman, in his introductory chapter, that the 

 canals on "the head are innervated mainly from the fifth pair," it may be inferred that 

 he has not directed his attention specially to the nerve-supply of the canals ; the fifth, as 

 I have already indicated, taking no part in innervating the canals. To admit of the old 

 and complex nomenclature being compared with the simpler one I propose to use, I have 

 reproduced Garman's figures of Laemargus (Somniosus — woodcuts A and B), and will in- 

 dicate, as I proceed, which of his canals correspond to the cranial canals I have already 

 mentioned, viz., the supra- and infra-orbital and hyomandibular canals. 



In referring to the development of the lateral sense organs, it was mentioned that 

 while the superficial portion of each epidermic thickening gives rise to the rudiment of a 

 branchial sense organ, the deep portion assists in forming a cranial ganglion. According 

 to Beard (27), there should be in a typical Selachian seven dorsal (supra-branchial) nerves, 

 and a corresponding number of sense organs — i.e., (1 and 2) an ophthalmicus profundus 

 and an ophthalmicus superficialis of trigeminus in connection with sense organs over the 

 snout ; (3 and 4) an ophthalmicus superficialis of facial in connection with the supra- 

 orbital, and a ramus buccalis of facial in connection with the infra-orbital sense organs ; 

 (5 and 6) the glossopharyngeus and anterior vagus branches in connection with the supra- 

 temporal sense organs ; and (7) the nervus lateralis of vagus with the organs of the 

 lateral line. Whether this supposed typical arrangement obtains in embryo SelachiaDS 

 remains to be seen ; but I have not found that Beard's scheme holds for either fully 

 developed sharks or rays, and according to Allis it does not hold in the case of Amia. In 

 Amia, Allis states that " the trigeminal and ophthalmicus profundus take no part with 



