192 



ZOOLOGY 



coalesced neural arches already referred to. The remaining fins 

 are formed quite on the Elasmobranch type, as is also the shoulder 

 girdle. The right and left halves of the pelvic arch are separate 

 from one another, being united in the middle ventral line by 

 ligament only ; each presents a narrow iliac region and a broad 

 flat pubo-ischial region perforated by two apertures or fenestree 

 closed by membrane, one of them of great size in Callorhynchus. 



ja.s.c 



Fig. 84fi. — Chixnaera monstrosa, lateral view~of .skull, a. s. c. position of anterior semi- 

 circular canal ; c. hy, ceratohyal ; e. hy. epihyal ; Jr. cl. frontal clasper ; h. s. c. position of 

 horizontal semicircular canal ; i. o. s. interorbital septum ; Ih. 1, lb. 2, lb. 3, labial cartilages ; 

 Mck. c. mandible ; Nv. ^, optic foramen ; iVr. 10, vagus foramen ; olf. cp. olfactory capsule ; 

 02). r. opercular rays ; pal. qu. palatoquadrate ; ph. hy. pharyngohyal ; p. a. c. position of 

 posterior semicircular canal ; qu. quadi'ate region ; r. rostrum. (After Hubrecht.) 



The skeleton of the anterior clasper articulates with the pubic 

 region. 



Digestive Organs. — The teeth (Fig. 850) are very cliaracter- 

 istic, having the form of strong plates with an irregular surface 

 and a sharp, cutting edge. In the upper jaw there is a pair of 

 small vomermie teeth (vo. t.) in front, immediately behind them 

 a pair oi la.Tge palatine teeth (pal. t.), and in the lower jaw a single 

 pair of large mandibiolar teeth (mnd. t.). They are composed of vaso- 

 dentine, and each palatine and mandibular tooth has its surface 

 slightly raised into a rounded elevation of a specially hard sub- 

 stance, of whiter colour than the rest of the tooth, and known 

 as a tritor (tr.). The stomach is almost obsolete, the enteric canal 

 passing in a straight line from gullet to anus ; there is a well- 

 developed spiral vahe in the intestine. 



Respiratory Organs. — There are three pairs of holobranchs or 

 complete gills borne on the first three branchial arches, and two 

 hemih^anchs or half-gills, one on the posterior face of the hyoid, 



