xm PHYLUM CHORDATA 161 



region for the hyo-mandibular is sometimes borne on a projecting 

 process, sometimes on the general level of the lateral surface. 

 Sometimes in the Rays there is a smaller articulation behind for 

 the first branchial arch. 



The upper and lower jaws the palato-quadrate and MeckeVs car-, 

 tilagi are connected with the skull through the intermediation 

 of a hyo-mandibular cartilage (Fig. 768, liy. mn. ; Fig. 788, h. m.). 

 In the Sharks the palate-quadrate has a process (absent in the 

 Ravs ) for articulation with the base of the skull. In Hexanchus 



, 



and Heptanchus (Fig. 789) there is a prominent post-orbital pro- 

 cess of the palato-quadrate for articulation with the post-orbital 

 region of the skull (amphistylic arrangement). At the sides of 



'. or If 



Fir;. 780. Lateral view of the skull of Heptanchus. ruck. Meckel's cartilage ; pal.-qu. palato 

 quadrate ; pt. orb. post-orbital process of the cranium, with which the palato-quadrate 

 articulates. (After Gegenbaur.) 



the mouth are a series of labial cartilages, usually two pairs 

 above and one pair below. Attached to the hyo-mandibular is 

 a thin plate of cartilage the spiracular (Fig. 788, sp.) which 

 supports the anterior wall of the spiracle. The hyoid arch is in 

 most of the Elasmobranchs connected at its dorsal end with the 

 hyo-mandibular, sometimes at its distal extremity, sometimes near 

 its articulation with the skull ; but in some Rays it is not so 

 related, but articulates separately and independently with the 

 skull behind the hyo-mandibular, and in the genera Hypnos and 

 Trygonorliina it articulates with the dorsal portion of the first 

 branchial arch. In the Sharks the hyoid is usually relatively 

 massive ; in the Rays it is smaller, and in most cases closely 

 resembles the branchial arches, and bears similar cartilaginous 

 rays ; a larger or smaller median element, or basi-hyal, is present 

 in all cases. 



VOL. II M 



