SKULL — ELASMOBRANCHS 



87 



confined to the margins, but in a few forms (feeding on hard-shelled 

 animals) extending some distance in the mouth. The attachment 

 of the pterygoquadrate to the cranium varies in different groups. 

 Notidanids have a strong process (especially strong in Heptanchus, 

 fig. 90) on the upper margin of the pterygoquadrate which articu- 

 lates with the side of the postorbital crest, and in addition this 

 element is connected by ligaments with the hyoid arch and thus 

 indirectly with the cranium (amphistyly) . In most species the 

 pterygoquadrate is connected in front with the cranium by ligaments, 

 while behind, the anterior angle of the hyomandibula intervenes 



Fig. 90. — Skull of Heptanchus maculatus (Daniel, '16). hr, hyoid rays; w, Meckel's 

 cartilage (lower jaw); pq, pterygoquadrate; 2-4, nerve exits. 



between cranium and upper jaw (hyostyly), this reaching its extreme 

 in skates where the hyomandibula is wholly suspensorial. Each 

 half of the pterygoquadrate may have a palatal process on its upper 

 margin which slides on the palatobasal surface (p. 85) of the 

 cranium, tending to prevent disarticulation of the jaws by the 

 struggles of the prey. In skates the suspension of the jaws is wholly 

 hyomandibular, the anterior ligaments being lost, thus allowing the 

 protrusion of the jaws in seizing food. 



The Holocephali differ from all other sharks in the attachment of the jaws. 

 In the young the pterygoquadrate is free from the cranium, but fuses early with 

 its wall (fig. 91, B) so completely that no trace of a suture persists in the adult. 

 This autostyly is strikingly similar to conditions in higher groups. 



Sharks have from one to three spiracular cartilages (supporting the spiracular 

 gill) between the hinder end of the pterygoquadrate and the hyoid. These are 

 sometimes regarded as remnants of a lost visceral arch. These cartilages in 

 skates are more closely related to the anterior side of the mandible. 



