196 



CHORDATA. 



upper ends are frequently toothed and, in chewing are opposed by the 

 rudimentary last arch, on which account these are spoken of as the 

 superior and inferior p/iaryngeal bones. The anterior visceral arches are 

 greatly different in cartilaginous and bony fishes. In the former (fig. 546) 

 the pterygoquadrate (pq) and the Meckelian cartilage (Md) bear teeth 

 and oppose each other in biting. In the bony fishes (fig. 547) the 

 teeth of the lower jaw oppose the tooth-bearing elements, premaxillary 

 and nuL\illar\, of the maxillary series, while the pterygoquadrate ele- 

 ments the palatine and the series of ptery golds are the antagonists of 

 the hyoid. In the elasmobranch type the two halves of the pterygo- 



ob ic 



17. <?/i ff jto Tr 



Jt. 



FIG. 546. Cranium, visceral arches, and part of vertebral column of Mustelus 

 vulqaris. ao, antorbital process; co, copula; gp, foramen for glossopharyngeal ; H, 

 otic capsule and hyoid; Hm, hyomandibular; ic, intercalare; Md, mandible (Meckel s 

 cartilage); N, nasal capsule; o, optic foramen; ob, neural arch; po, postorbital process; 

 Pq, pterygoquadrate; ps, spinous process; R, rostrum; r, ribs; tr, trigeminus foramen; 

 v, vagus foramen; 1-8, visceral arches; i, labial; 2, mandibular; 3, hyoid; 4-8, gill arches. 



quadrate meet by symphysis in the middle line; in the others they are 

 separated by the floor of the skull. 



A second characteristic of the bony fishes is already outlined in the 

 cartilaginous fishes: the modification of the hyomandibular to a suspensor 

 of the jaws. In the elasmobranchs (especially the skates) the parallelism 

 of hyoid and mandibular arches is lost, the hyomandibular separating 

 from the hyoid and attaching itself to the hinge of the jaws. In the 

 teleosts the hyomandibular is thus brought in connexion with the quad- 

 rate, and lies between it and the cranium, the joint being thus indirectly 

 supported from the cranium, a bone, the symplectic (known only in fishes) 

 helping out the suspensor, while another bone, the interhyal, connects 

 this with the hyoid, which itself divides into two, so that the hyoid arch, 

 like a gill arch, consists of four elements. 



