70 PROFESSOR J. C. EWART ON THE 



The infra-orbital canal has a simpler course than in most Selachians, and the absence 

 of the suborbital loop, found, for example, in Acanthias, is especially noteworthy. 



Allis considers the infra-orbital canal of Amia as the main canal of the cranial 

 system. He describes it as being " directly continuous with the lateral canal." i.e., as 

 extruding in Amia to the posterior boundary of the cranium (21, fig. 3, PI. II.) — some 

 distance beyond the " supra-temporal cross commissure" (lc., fig. 3). 



The infra-orbital canal of Allis is thus something more than the canal of the buccal 

 nerve, for it receives branches from the glossopharyngeal and vagus nerves ; and this 

 being the case, it must presumably have originated from parts of three branchial sense 

 organs. It seems to me most desirable in dealing with the canals, to be guided as far as 

 possible by their innervation. Hitherto the canal of the lateral line has been usually 

 described as terminating at the anterior end of the trunk, i.e., as not extending into the 

 head region ; but, seeing that the lateral canal is supplied throughout by a cranial nerve, 

 I fail to see why it may not be considered as extending into the head region. If it is 

 right to consider the supra-orbital canal as co-extensive with branches of the ophthalmicus 

 superficialis nerve, and the hyomandibular with branches of the hyomandibular nerve, in 

 other words, to consider the whole of the canals, or portions of canals, developed in 

 connection with any given nerve as forming one system, it is only logical to look upon 

 the whole of the canal or canals innervated by the lateralis nerve as forming one system, 

 and to consider the infra-orbital canal as coming to an end at the point where it ceases 

 to be supplied by the buccal nerve. Following this plan in the case of Amia, I would 

 describe the infra-orbital canal as beginning at the upper end of the hyomandibular canal 

 (HM. 1 , fig. 3), and extending forwards and then downwards under the eyeball, after 

 communicating with the supra-orbital. If, as seems probable, the superficial ophthalmic 

 and buccal nerves and their ganglia have resulted from the splitting of a single nerve, it 

 might be more accurate, in the case of Amia, to consider the canal between the upper 

 end of the hyomandibular canal and the point at which the supra- and infra-orbital canals 

 separate from each other as a special (say otic) portion (ot., fig. 3) — formed from an 

 unsplit part of an embryonic sense organ — more especially as in Amia it is supplied by a 

 nerve (ot.n., fig. 3) which springs, according to Allis, directly from the facial ganglion. 



The short portion (T., fig. 3) in Amia, between the upper end of the hyomandibular 

 (operculo-mandibular of Allis) and the supra-temporal cross commissure (lc, fig. 3), 1 

 would describe as the temporal canal or canal of the glossopharyngeal nerve ; and the 

 whole of the canal behind this, on to the end of the trunk, including the supra-temporal 

 cross commissure, I would describe as the lateral canal — the canal of the lateralis nerve. 

 If in the ancestral forms there was but a single cranial canal, as there is now only a 

 single canal in the trunk, it is possible that the single ancestral cranial canal is now 

 represented by the infra-orbital canal. This is a point, however, that may be made clear 

 when the development of these canals is worked out. Assuming that the infra-orbital 

 is the main canal of the head, as the lateral is the main canal of the trunk, the supra- 

 orbital would require to be looked upon as a dorsal offshoot from the infra-orbital ; and 



