66 PROFESSOR J. C. EWART ON THE 



any of their branches in the innervation of the canals or pit organs," and that " there is a 

 large and important operculo-mandibular line of organs," in the innervation of which 

 none of the supra-branchial branches given in Beard's scheme take any part (30). 



. These statements as to Amia apply also to Lsemargus, and the only essential difference 

 in the innervation of the canals in the two, in many respects, diverse forms, is that in 

 Lsemargus I have not succeeded in finding a branch from the glossopharyngeal nerve to 

 any of the sense organs. 



In view of the statement that the embryonic epidermic thickening assists in forming 

 a cranial ganglion as well as a branchial sense organ, it is worth specially noting that 

 each of the four nerves which sends branches to the sensory canals, i.e., the ophthalmicus 

 superficialis, buccal, and hyomandibular divisions of the facial, and the lateralis 

 division of the vagus, presents some distance from its origin a well-marked ganglionic 

 swelling crowded with large nerve cells. In the skate these ganglia are relatively as 

 large as or larger than the ganglia on the profundus, trigeminal, and glossopharyngeal 

 nerves — nerves which in adult sharks and rays have not yet been found in connection 

 with lateral sense organs. It is further worthy of note that the buccal and superficial 

 ophthalmic ganglia are sometimes in contact with each other by their proximal ends. 

 From this it might be inferred that there has been a splitting of the original epidermic 

 thickening above the spiracular cleft, the splitting resulting not only in the formation 

 of two ganglia, but also of two sensory canals — the supra-orbital above, and the infra- 

 orbital below, the eyeball. 



III. The Sensory Canals of Lsemargus. 



The position of the four main canals already mentioned is indicated by their names. 

 The supra-orbital canal (S.O., fig. 1, PI. I.) begins some distance in front of the auditory 

 pores and extends forwards towards the tip of the snout, which it perforates, and then 

 runs backwards to unite with the infra-orbital as it bends inwards towards the middle 

 line. 



The infra-orbital (less the accessory otic portion ; I.O., fig. 1), beginning in connection 

 with the supra-orbital, runs outwards behind the eyeball, and then forwards between the 

 eyeball and the mouth, and after communicating with the supra-orbital it bends inwards 

 and forwards towards the tip of the snout. 



The hyomandibular (HM., fig. 1), communicating in front with the infra-orbital, runs 

 backwards in a nearly horizontal direction. 



The laterals (L., fig. 1) continuous in front with the otic portions of the infra- 

 orbitals, after communicating with each other behind the auditory pores, run almost 

 directly backwards, one at each side of the trunk, until on a level with the posterior 

 margin of the lower lobe of the caudal fin, when they bend upwards to terminate on the 

 upper lobe of the fin, one at each side of the terminal portion of the vertebral column. 



1. TJie Supra-orbital Canal. — This, the canal of the superficialis ophthalmicus 



