44 THE ELASMOBRANCH FISHES 



exceedingly thin-walled and open to the exterior by the nasal apertures. Sur- 

 rounding the aperture is the arch-like nasal cartilage ( n.c.) . The optic capsule 

 does not fuse with the cranium. 



On each side at the posterior third of the cranium is the auditory capsule 

 (a.c, fig. 47) in which the semicircular canals and the organs of hearing are 

 located. Between the auditory and the nasal capsule is the large orbit for the 

 eye. Overhanging this is the supraorbital crest {s.o., fig. 48) , the anterior pro- 

 jection of which is the preorbital (pr.o.), and the posterior one the postorbital 

 process {po.o.). Ventral to the posterior part of the orbit the cranium bends 

 sharply downward forming the basal angle {h.a., fig. 47) . On the anterior face 

 of the basal angle is a flattened articular surface against which the orbital 

 process of the upper jaw plays. Anterior to the basal angle and extending 

 from the margin of the cranium is an antorbital process {a.pr., fig. 47). This 

 process is called by AUis (1923) and by others the ectethmoidal process. 



Numerous foramina through which nerves and blood vessels course perfo- 

 rate the walls of the cranium. The first of these between the orbit and the nasal 

 capsule is the anterior opening of the orbitonasal canal (o-n.), its posterior 

 opening {o-7i}) lying in the anterior angle of the orbit. Above this opening is 

 a smaller foramen for the anterior cerebral vein (f.a.c). Ventrally and at the 

 middle of the orbit is the large optic foramen (f.II) through which the second 

 cranial nerve reaches the brain. Directly above it is the ophthalmic fora- 

 men through which the superficial ophthalmic branch of the seventh nerve 

 (f.o.VII) leaves the orbit. A short distance ventral and anterior to this is a 

 small opening through which the deep branch of the fifth nerve leaves the 

 orbit. Posterior to the ophthalmic is the small trochlear foramen (f.IV) for 

 the fourth cranial nerve which passes to the superior oblique muscle of the 

 eye. Behind the optic and below and slightly back of the tip of the postorbital 

 process is the large orbital fissure (o.f.) ; through this the fifth, and a part of 

 the seventh cranial nerves pass. The sixth nerve enters the orbit through its 

 own foramen in the anteroventral margin of the orbital fissure. Below and 

 slightly posterior to the orbital fissure is the facial foramen (f.VIP) for the 

 hyomandibular branch of the seventh nerve. On a line between the facial and 

 the optic foramina are two perforations, the larger and posterior of which is 

 for the interorbital canal (i.o.) by means of which the blood sinuses of the two 

 orbits communicate. The other of these perforations (f.r.a.) is for the entrance 

 of the ramus anastomoticus arter^^ Above this is the small opening {f.III) for 

 the exit of the third cranial nerve to muscles of the eye. 



VISCERAL SKELETON 



The visceral skeleton is composed of a series of cartilaginous arches which 

 more or less completely surround the buccal cavity and the pharynx. The num- 

 ber of these in Heptanchus (nine) exceeds that of any other present-day Elas- 

 mobranch. The arches may be divided into two groups. The first group com- 

 prises the mandibular and the hyoid arches, each of which is made up of two 

 segments. The second group consists of seven branchial arches which support 



