60 THE ELASMOBRANCH FISHES 



joins the nasal capsule and the posterior part extends back to the postorbital 

 process (po.o.). Between anterior and posterior parts is located the secondary 

 orbit (or.). 



In the notidanid sharks, in Chlamydoselachus, and in the rays there is a 

 posterior projection from the preorbital region, the antorbital process (figs. 

 69 and -47, a.pr.) . In the notidanids and Acanthias this serves for the attach- 

 ment of one of the superior labial muscles. In the rays it unites the pectoral fin 

 skeleton to the cranium. 



The eyeball is held out from the cranial wall by a rod, the optic pedicel 

 (Heterodontus, fig. 67, o.p.) ; this pedicel in some of the sharks has a terminal 

 expansion into which the eyeball fits. In Chlamydoselachus (fig. 46) it further 

 serves as a place of origin for the rectus muscles. In the rays the pedicel is 

 plate-like and may be fixed to the eyeball (Torpedo). 



The apertures which perforate the orbital region for the cranial nerves and 

 blood vessels vary considerably in size and position from those given for Hep- 

 tanchus. Ordinarily the optic foramen (/.//, fig. 66) is relatively large and 

 occupies a central position in the orbit, but in types like Rhinohatis (fig. 68) 

 it is well forward. In the embrj-o (Acanthias) this foramen separates the 

 alisphenoid from the trabecular cartilage. The oculomotor and the troch- 

 learis foramina take positions respectively behind and above the optic; but the 

 trochlearis is variable. In the rays (Rhinohatis, fig. 68, f.IV; Myliohatis) it is 

 above but posterior to the aperture for the optic nerve. The orbital fissure 

 (o.f.) usually gives exit to the fifth and a considerable part of the seventh 

 cranial nerves, and in a type like Rhinohatis is of unusual size. In this type it 

 is not unlike the large fissure which early forms a separation of the alisphen- 

 oids from the parachordal cartilages in the embryo of Acanthias. In Mustelus 

 henlei, where there are special foramina for the superficial division of the 

 seventh nerve and the profundus division of the fifth nerve, the fissure is re- 

 duced in size. The foramen for the sixth nerve usually opens separately at 

 the base of the orbital fissure. The facial foramen for the hyomandibular 

 branch of the seventh nerve may be located posterior to the orbit, as in Hep- 

 tanchus (f.VII', fig. 47), in Scymnus, and in other forms; or it may be in the 

 posteroventral angle of the orbit (Heterodontus, fig. Q>Q, f.VII; and others). 

 In the upper anterior angle of the orbit is the ophthalmic foramen (or fora- 

 mina) which gives exit to the ophthalmicus superficialis of the seventh nerve. 

 In Heterodontus, as is general for the Elasmobranchs, the profundus nerve 

 (f.op.) leaves the orbit by an extra foramen ventral to that for the ophthal- 

 micus superficialis. 



Anterior to the orbit is the ethmoidal region in which is situated the nasal 

 or olfactory capsule. The capsules in the simpler forms are more or less ter- 

 minal in position, while in the highly specialized rays, as for example Mylio- 

 hatis, the region is bent sharply downward so that the apertures are entirely 

 ventral in position. The olfactory cups are usually more or less surrounded by 

 cartilage, leaving their apertures as relatively small openings. These openings 

 are visible in side view in the sharks only. The nasal cartilages surrounding 

 the openings have been described. 



