64 PROFESSOR J. C. EWART ON THE 



principal cranial canals have been formed in connection with three extensions of the 

 facial — two forwards and one outwards. One of these divisions of the facial (the 

 ophthalmicus superficialis) has grown forwards above the eyeball, another (the buccal) 

 has grown forwards under the eyeball, and the third (the hyomandibular) outwards and 

 forwards behind the spiracle. The three canals in relation with these nerves should have a 

 corresponding position ; one should run forwards above the eyeball, one curve downwards 

 and forwards below the eyeball, and a third should lie behind the spiracle. 



If we turn to Lsemargus we first of all notice, all over the head, but especially above 

 and below the snout, a large number (over 1500) of small but perfectly distinct pores. 

 The majority of the pores have the margin slightly projecting and deeply pigmented. 

 Without much difficulty it becomes evident that some of the pores serve as openings 

 for ampullary canals ; while others lead by short tubules into sensory canals. If the 

 arrangement of the openings of the sensory tubules is studied, or if the canals from which 

 the tubules spring are exposed, it will be found that, as expected, one canal extends 

 above, while a second lies below the eyeball. The first, which is related to the 

 ophthalmicus superficialis nerve, maybe known as the supra-orbital canal (S.O., figs. 1 

 and 2). The second, which is related to the buccal nerve, may be known as the infra- 

 orbital (I.O., figs. 1 and 2). But although the canals of the ophthalmic and buccal 

 divisions of the facial are easily recognised in Lsemargus, there is some difficulty 

 in distinguishing the canal related to the hyomandibular division. This canal, though 

 not at first evident in Lsemargus and other Selachians, is easily distinguished in the 

 ganoid Amia. By referring to figure 3 (PI. II.), a canal (HM.) as complete and distinct 

 as the infra-orbital (I.O.) will be seen beginning on the same level as the infra- orbital, 

 and extending downwards and forwards along the mandible. This canal is supplied 

 throughout its entire length by the hyomandibular division of the facial, and it has the 

 same relation to the hyomandibular as the infra-orbital canal has to the buccal division 

 of the facial. 



I propose to call this canal the hyomandibular canal. Allis (30) describes it as the 

 operculo-mandibular ; but this name could not well be used for either sharks or rays, in 

 which, though the lower part of the canal is usually more or less complete, the proximal 

 part is never, so far as I know, present. 



In the remarkable fish Chlamydoselachus (33), the mandibular portion (0.), as shown 

 in figure 4, is complete, and, in addition, there is an extension forwards (ang.) to join 

 the infra-orbital and another backwards (j.) over the operculum, which bifurcates near its 

 margin, one division running upwards and forwards (sp., fig. 5), the other downwards 

 and forwards {g., fig. 4), to join the mandibular portion. 



In Lsemargus, the lower (mandibular) as well as the upper part of the hyomandi- 

 bular canal is absent ; and the only representative of the long canal of Amia is a short 

 horizontal canal (HM., fig. 1, PI. I.) which runs backwards from the infra-orbital, external 

 to a well-marked fold of skin at the side of the mouth. 



That this short canal belongs to the hyomandibular and not to the infra-orbital, 



