SKULL AND VISCERAL SKELETON OF THE GREENLAND SHARK. 289 



the greater portion of the basal area for the posterior half of the skull. Sometimes the 

 occipital part of this plate is irregular, and has processes standing out from it. 



Pneumogastric Canals. — The outer opening of each canal (Fig. 1, Yg f ) is large 

 and funnel-shaped, and is placed at the back of the skull, external to an occipital 

 process. Each canal pursues a course from within backwards and outwards. Beside 

 each pneumogastric canal are two canals, with a similar direction, for the first and second 

 spinal nerves, and their outer orifices are placed near each other, internal to the pneumo- 

 gastric foramina. In the skulls in which I followed up these canals I noticed that in 

 part of their course they communicated with the pneumogastric passage. 



Gegenbaur describes in Hexanchus and in other Selachians a canal, the orifice of 

 which is situated on each postero-lateral edge of the auditory region, near the foramen of 

 exit for the glosso-pharyngeal and pneumogastric nerves. These canals he describes 

 with the occipital region, since each opens into a pneumogastric canal. A vein which 

 he considers to be the primitive jugular vein passes along each of these canals. In 

 Lsemargus I do not find this canal, but I notice that a vein which opens into the anterior 

 cardinal sinus issues from the pneumogastric canal in company with the nerve. If this 

 vein corresponds to that described by Gegenbaur as passing along a separate canal for 

 part of its extent, it may be concluded that in Lsemargus this canal has blended with 

 that for the pneumogastric nerve. 



The Auditory Region. 



This region is continuous with the occipital region behind, and with the orbital in 

 front. The pneumogastric canals mark its limit behind, and at its fore part are the wide 

 passages for transmitting the trigeminal and other nerves. The external configuration 

 of this region is little affected by the organ of hearing which it contains, the semicircular 

 canals and vestibules giving rise to no such elevations as are so characteristic of some 

 Selachian skulls. 



The dorsal aspect of this region presents on either side a crest, internal to which is a 

 shallow groove on which are several foramina. These lateral crests, which project 

 outwards and upwards, commence in front in a small eminence, and are continued from 

 this point backwards to the hinder part of the auditory region. The central part of 

 the dorsal surface is slightly raised, and the parietal fossa (Figs. 1 and 5, P) is situated 

 here. This fossa is deepest posteriorly, and in this position the vestibular aqueducts 

 open. The floor of the fossa slopes gently upwards towards the surface of the skull, and 

 its lateral edges becoming more prominent as they pass backwards, meet at its hinder 

 part, in a small elevation (Pm). 



The ventral portion of the auditory region forms a considerable portion of the 

 basilar plate (Fig. 3, Bp) of the skull. A groove (C^), for a carotid artery, beginning 

 on each side about the anterior third of the lateral edge of this plate, runs forwards 



