xin PHYLtfM CHORDATA 193 



the other on the anterior face of the fourth branchial arch. The 

 fifth branchial arch is, as usual, gill-less, and there is no cleft 

 between it and its predecessor. The gill-filaments are fixed in 

 their whole length to an interbranchial septum, as in Elasmo- 

 branchs. 



The small heart resembles that of the Dog-fish in all essential 

 respects, being formed of sinus venosus, auricle, ventricle, and 

 conus arteriosus, the last with three rows of valves. 



Nv. slo' 



or JV'v. So 



■vo.l 



Fig. S50.— Callorhynctaus antarcticus, sagittal section of skull ; the labial cartilages are 

 removed, a. s. c. apertures through which the anterior semicircular canal passes from the 

 cranial cavity into the auditory capsule ; e. t, d. aperture for endolymphatic duct ; mck. c. 

 Meckel's cartilage ; mnd. t. mandibular tooth ; nch. notochord ; Nv. 5, trigeminjil foramen ; 

 Nv. 5. 0, foramen for exit of ophthalmic nerves , Nv. 5.'o', canal for ophthalmic nerves with 

 apertures of entrance and exit ; Nv. 10, vagus foramen ; oc. en. occipital condyle ; or. 

 fenestra separating cranial cavity from orbit ; pal. qu. palatoquadrate ; pat. t. palatine tooth ; 

 pn. position of pineal body ; pt. pit for extra-cranial portion of pituitary body ; p. s. c. apertures 

 through which the posterior semicircular canal passes into the auditory capsule ; lu. quadrate 

 region of palatoquadrate ; r. rostrum ; sac. depression for sacculus ; s. t. sella turcica ; ir. 

 tritor ; vo. t. vomerine teeth. 



The brain (Fig. 851), on the other hand, is very unlike that of 

 Scyllium, but presents a fairly close resemblance to that of 

 Scymnus. The medulla oblongata {med. ohl.) is produced laterally 

 into large frill-like restiform bodies {cp. rst.), which bound the hinder 

 halfof the cerebellum {Mm.). The diencephalon ((^mw.) is extremely 

 long, trough-shaped, and very thin-walled, without pronounced 

 optic thalami ; it is continued without change of diameter into a 

 distinct prosencephalon, which gives off the cerebral hemi- 

 spheres (crb. h.) right and left. The combined diacoele and proso- 

 ccele {di. cce.) are widely open above in a brain from which the 



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