SKULL AND VISCERAL SKELETON OF THE GREENLAND SHARK. 295 



For descriptive purposes the cranial cavity may be described as consisting of a posterior, 

 middle, and anterior division. The posterior portion extends from the foramen magnum 

 to the hinder wall (Ds) of the pituitary fossa ; the middle from this point forwards to the 

 optic foramina (0) ; and the anterior division lies in front of these openings. 



The posterior division. — This extends, as just stated, from the foramen magnum to the 

 hinder wall or dorsum sella? (Ds) of the pituitary fossa. In some sharks, e.g. Hex- 

 anchus, the dorsum sella? forms a considerable elevation on the cranial floor. Between 

 this elevation and the foramen magnum there is a hollow, but in Lsemargus, as the dorsum 

 sella? rises very slightly on the floor of the cavity, the hollow between it and the foramen 

 magnum is very shallow. A distinct elevation passes from the dorsum sella? for a short 

 distance up the cranial wall, and behind it the large foramen (Tr) for the trigeminal, 

 abducens, and part of the facial nerve is situated, while the foramen (Om) for the oculi- 

 motor, which belongs, however, to the middle cranial region, is placed in front of it. 

 There is a depression (Af) a short distance behind the foramen for the trigeminal and 

 other nerves, at the bottom of which are two foramina, an anterior for the portion of the 

 facial nerve which does not pass through the trigeminal canal, and a posterior for the 

 auditory nerve. The foramen (Grp) for the glosso-pharyngeal nerve is situated some 

 distance behind the depression common to the two nerves just mentioned. The pneumo- 

 gastric foramen (Vg), which has a funnel-shaped depression leading up to it, is situated 

 behind that for the glosso-pharyngeal, but at a higher level. A small foramen (Si) for 

 the first spinal nerve lies below the pneumogastric opening, that for the second spinal 

 (S2), which is larger, is placed behind it, while the third spinal nerve passes through a 

 canal (S3) situated between the skull and the arch of the first vertebra. In having 

 apertures and canals for the first and second spinal nerves in the cranial wall, La?margus 

 agrees with Scymnus, Acanthias, and some other sharks. 



A downward projection from the roof of the cavity, caused by the sinking of the floor 

 of the parietal fossa (P), presents itself in this part of the cranium. From this projection 

 the roof passes forwards and upwards to its highest elevation. 



The middle division. — This extends from the dorsum sella? (Ds) as far as the optic 

 foramina (0), and its vertical diameter is greater than that of the division behind 

 or before it. 



The pituitary fossa. — This fossa occupies the floor of the division, and extends from 

 the dorsum sella? (Ds) to (M) its interior limit. The hinder part of the fossa is deep, 

 and its anterior wall slopes upwards and forwards and terminates in the slight sinking 

 (M) on the cranial floor. The hinder and postero-lateral walls of the fossa are especially 

 steep. Several canals open at various points on the walls of the fossa. 



The carotid canals. — These two canals begin (Fig. 3, Qa'), as already seen, at the 

 inner ends of the grooves (Cg) on the under surface of the basilar plate of the skull, and 

 pass through the cranial floor, and meet each other a short distance behind the pituitary 

 fossa. From their point of union a short wide canal passes forwards and slightly upwards 

 to open (Fig. 5, Ca) on the hinder wall of the fossa at its lower part. 



VOL. XXXVII. PART II. (NO. 15). 2 U 



