xiii 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- 

 tilage are connected with the skull through the intermediation 

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

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

 Rays) 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 



ttt. or I/ 



f^\TrJ 

 f 

 t 





u 

 me It 



KKi. 7S(. Lateral view of the skull of Heptanchus. nick. 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 

 Trygonorhina 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. IT M 



