68 PROFESSOR J. C. EWART ON THE 



depth rapidly increases as it runs forwards to perforate the snout. The ventral portion 

 lies at an average depth of 5 mm. ; but the part in front of the snout is deeper, while 

 the part over the nasal capsule is immediately under, or actually embedded in, the skin. 

 The supra-orbital canal contains numerous sense organs, and is perforated by two rows of 

 pores or apertures, an inner row for the nerves which reach and end in the sensory 

 hillocks, and an outer row of larger apertures, which lead into the tubules by which the 

 canal communicates with the exterior. The sense organs and tubules are shown on the 

 right side of figure 1. 



Eighty-three tubules, and a corresponding number of sense organs, were found in the 

 supra-orbital canal of the specimen examined. Each organ received a delicate branch 

 of the ophthalmicus superficialis nerve, the nerves entering by the minute pores nearly 

 opposite the inner ends of the tubules. 



The length of the tubules varies almost constantly with the distance of the canal 

 from the surface. Some of them are under 2 mm. in length, while others are nearly 

 1 cm. The majority of the tubules run obliquely outwards ; but a number proceed from 

 the outer surface of the canal directly through the skin. The openings of the tubules 

 are from 1 to 1'5 mm. in diameter ; and the outer 2 or 3 mm. are distinctly pigmented. 

 The openings of the tubules are readily distinguished from the openings of the ampullary 

 canals. They are more regular in their arrangement than the openings of the ampullary 

 canals, and the margins are usually more deeply pigmented. 



By comparing the supra-orbital canal of Lsemargus with that of Chlamydoselachus 

 (cr., r., sr., figs. 4 and 5), it will be noticed that it differs in several respects. Not only 

 is it not in a line with the lateral canal as is the case in Chlamydoselachus, but it neither 

 directly nor indirectly communicates with this canal ; and, further, unlike all the ordinary 

 Selachians, it returns to the dorsal surface of the snout as it proceeds backwards to join 

 the infra-orbital. This return to the dorsal surface seems to be due to the nasal opening 

 occupying a more outlying position than is usually the case. Posteriorly, the two supra- 

 orbitals bend inwards towards each other, and thus approach the arrangement in the 

 Holocephala, in which the connection between the canals of the two sides is accomplished 

 by the union of the supra-orbitals instead of the union of the laterals behind the auditory 

 pores. 



Innervation of the Supra-orbital Canal. — The nerve (the superficial ophthalmic of 

 facial (s.o.f., fig. 1) of the supra-orbital canal gives off branches as it runs forwards over 

 the eyeball to the first (cranial) part of the canal. On reaching the nasal capsule, it 

 sends a branch downwards and outwards (s.of. 2 ,"B.g. 1) to supply the sense organs of the 

 ventral part of the canal ; and, then, as it passes forwards towards the end of the snout, it 

 gives off twigs for the sense organs in the part of the canal in front of the nasal capsule. 

 A more detailed account of the innervation of this and the other canals will be included 

 in a paper, in course of preparation, on the cranial nerves of Lseniargus. 



2. The Infra-orbital Canal. — This canal (I.O.-I.O. 5 , fig. 1), as already mentioned, 

 communicates at its origin with the supra-orbital. From the common opening it runs out- 



