294 MR PHILIP J. WHITE ON THE 



Nasal-ring -cartilage (Figs. 2, 3, and 4, Na). — This is an incomplete ring of carti- 

 lage which surrounds the greater part of the nasal orifice. Externally it is in contact 

 with the free margin of the nasal capsule, and the two ununited ends of the ring are 

 directed inwards, the lower of these being very slender. Processes approach each other 

 from opposite points of the ring and thus divide the nasal orifice into an inner and outer 

 part, an arrangement which is not uncommon among Selachians. 



The pre-frontal fossa (Figs. 1 and 5, Tf). — This is an elongated deep fossa 

 which lies between the nasal capsules, and assists in separating them from each other. 

 The opening of the fossa on the roof of the cranium is somewhat elliptical, and its 

 margins, which are irregular and sometimes perforated at parts, curve downwards and 

 outwards to the upper surface of the nasal capsules, and terminate in front at the rostrum. 



The inter-nasal septum. — This septum separates the nasal capsules from each other 

 below. It is produced in front as a short truncated rostrum (Figs. 1-5, E), which 

 is broader in front than behind. The ventral portion of the septum is laterally 

 compressed and forms a keel (K), which runs forwards towards the rostrum. This keel 

 is especially prominent at its hinder part, and in this respect Lsemargus agrees with 

 Scymnus and Acanthias. 



The nasal fossa (Fig. 3, N/). — This is a deep fossa on the ventral surface, and lies 

 between the inner border of the posterior half of each nasal capsule and the inter-nasal 

 septum. These fossae also occur in some other Elasmobranchs. A canal which com- 

 municates with the cranial cavity opens at the bottom of each fossa at its hinder part. 

 A similar canal occurs in Heptanchus and some other forms, but not in Hexanchus. The 

 presence of this canal, together with other evidence, gives a clue to the origin of the three- 

 shanked rostrum found in some Selachians. A cartilaginous process (Fig. 3, P'), which may 

 be loosely connected with the nasal capsule, projects inwards and backwards under each 

 nasal fossa, and near the hinder part of the base of the process is the orifice (On") of a 

 canal which runs upwards and forwards to open into the hinder part of the nasal cavity. 

 The anterior opening of the orbito-nasal passage (On') lies a short distance behind the 

 posterior opening of the canal just noted, and a short groove leads from one to the other. 



Vertical longitudinal Section of the Skull in the mesial plane (Fig. 5). 



This section shows, — the continuation of the cartilage of the first vertebral centrum 

 with the skull, the thickness of the cranial roof and floor, the extent of the cranial 

 cavity, as well as that of the parietal and pre-frontal fossae. 



The cranial cavity. — This cavity, which is considerably larger than the brain which it 

 encloses, is open behind towards the neural canal, and is incompletely shut off from 

 the pre-frontal fossa (P/) by a cartilaginous partition (D) in front. On each side 

 this partition the cavity communicates with the nasal cavities through an olfactory 

 passage. 



