SKELETON 



71 



ventral halves (fig. 69, /), known respectively as the pterygo- 

 quadrate (palatoquadrate, ptgq), and Meckelian cartilages (w). 

 In the elasmobranchs and chondrosteous ganoids the pterygoquadrate 

 forms the upper jaw, lying parallel to and joined to the cranium by 

 ligaments or (chimaeroids) by continuous growth. With the appear- 

 ance of bones a new upper jaw is formed, as described below, and 



Fig. 71. — Ventral view or cranium and visceral arches of skate (Rata) after Gegen- 

 baur. cp. copula; h, hyoid; hm, hyomandibular; la, upper labials; mk, Meckelian 

 cartilage; nc, nasal capsule; pq, pterygoquadrate; r, rostrum. 



the pterygoquadrate becomes more or less reduced, and ossifies 

 as two or more bones with greatly modified functions. Meckel's 

 cartilage is the lower jaw of the lower vertebrates, while in the higher 

 it forms the axis around which the membrane bones of the definitive 

 jaw are arranged. 



The hyoid arch Hes between the spiracle and the first true gill cleft, 

 in the region of the seventh nerve. It divides into an upper element 



