SKULL — ELASMOBRANCHS 



83 



cartilage growths, to a plate in which ali- and orbitosphenoid parts 

 are distinguishable only by nerve exits, the arrangement of which 

 differs in different genera, but that of the second nerve indicates 

 approximately the posterior limit of the orbitosphenoid. The prootic 

 fissure behind the alisphenoid region contains the fffth and seventh 

 nerves, and is later converted into the foramen lacerum by the exten- 

 sion of the sphenolateral back to the otic capsule. 



Fig. 87. — Dorsal, ventral, lateral and sectional views of cranium of Acanthias 

 (Wells, '17). bs, basilar process; c, carotid foramen; ed, opening of endolymph duct; 

 ef, epiphysial foramen ;/<", endolymph fossa;//?, fossa hypophyseos;/w, foramen magnum; 

 nc, nasal capsule; op, foramen for ophthalmicus profundus; os, for branches of ophth. 

 superficialis; pd, for perilymph duct; r, rostrum; re, rostral carina; 2—10, nerve exits. 



A separate cartilage arises near the anterior end of the orbito- 

 sphenoid on the outer side and soon joins the cranial wall, forming 

 the antorbital process which separates the orbit from the nasal 

 capsule. The upper borders of the interorbital cranial walls extend 

 dorsally and medially and form the roof of the anterior part of the 

 cranial cavity, extending back to the synotic tectum, but leaving a 

 large gap, the anterior fontanelle, in front. These side walls also 

 grow forwards, forming the medial walls of the nasal capsule and the 

 large anterior projection (rostrum) of the cranium. The cranial 

 floor is completed in the same way, the medial growths joining the 



