SKULL AND VISCERAL SKELETON OF THE GREENLAND SHARK. 293 



front of that for the trigeminal and other nerves. The part of the skull through which 

 this canal runs is thick. 



Canal for the optic nerve. — This is a wide canal, and like the former passes almost 

 directly outwards through the skull wall. In a side view of the skull its outer orifice 

 (Fig. 4, 0') is seen lying considerably in front of that of the canal for the trigeminal and 

 other nerves. 



Canal for the patheticus. — This is a narrow canal, having from within a direction 

 outwards, downwards, and slightly forwards. Its outer opening (Fig. 4, Pa') is placed 

 at the upper part of the orbital cavity, and lies either directly above the optic foramen 

 or a little behind that point. 



Pre-orbital canal. — The hinder opening (Fig. 3, Pr') of this canal is situated at the 

 anterior and upper part of the orbit. The canal pierces the base of the pre-orbital process, 

 and runs from the orbital cavity to open (Fig. 1, Pr") on the roof of the cranium. A 

 second canal, which has already been noted, lies below it. 



Orbito-nasal canal. — This canal, which has its hinder orifice (Fig. 3, On) some 

 distance below that for the pre-orbital, passes forwards and slightly inwards, to open 

 (On') on the surface of the skull at the hinder and inner part of the nasal capsule. 



Inter-orbital canal. — The outer aperture (Fig. 4, Io') of this canal, the course of 

 which will be followed later, lies a little below the foramen of exit for the trigeminal 

 and other nerves. 



The Eye-stalk (PL I. Fig. 3, E. ; PI. II. Fig. 5).— This is an elongated rod, con- 

 tinuous at its proximal end (m) with the skull, and articulating at its distal extremity 

 (m') with the cartilaginous sclerotic of the eye-ball (PI. II. Fig. 6). The stalk is slender 

 at its proximal part, but increases in thickness towards its distal extremity, where it 

 presents a somewhat triangular cupped surface on which, in the recent state, the eye-ball 

 rests. The cartilage of which it is composed is similar to that composing the skull. 



The Ethmoidal Region. 



This region, which lies in front of the orbital, exhibits on either side the somewhat 

 flattened nasal capsules (Figs. 1 and 2, N) which form its lateral expansions. These 

 are separated from each other above by the deep pre-frontal fossa (Fig. 1, P/"), and below 

 by the inter-nasal septum. In front of the pre-frontal fossa the fore part of the region 

 is produced forwards as a truncated rostrum (Figs. 1-5, R). The long axes of the 

 nasal capsules have a direction forwards and slightly inwards. The ventro-lateral opening 

 of each capsule is situated anteriorly, and its margin, which is very irregular, supports a 

 ring-like nasal cartilage (Figs. 2, 3, and 4, Na). The ethmoidal canal (Fig. 4, em') is 

 found at the outer and lateral part of the dorsal surface of the capsule. A series of 

 longitudinal ridges, with narrow grooves between them, pass forwards and gently out- 

 wards on the upper aspect of the capsule. These ridges and grooves are especially 

 marked on the outer half of the roof of the capsule, and the latter lead to small canals 

 which open anteriorly near the free margin of the structure. 



