134 



VERTEBRATE SKELETON 



angle of the mouth far back. The articulare persists as cartilage 

 partly enclosed by the angulare, while farther in front the toothless 

 dentale envelops the anterior part of Meckel's cartilage. The tip 

 of the lower jaw is formed by the mentomeckelian already 

 mentioned. 



The hyobranchial cartilages (hyoid and four branchials) develop 

 in order from in front backwards. At first (fig. 143, A) the hyoid 

 and first gill arch are continuous, the others separate. The medial 

 ends of all the arches next fuse to a hypobranchial plate (B), the 

 copula between hyoid and the first gill-arch probably being basi- 

 branchial. At metamorphosis some parts are reduced, others 

 increased; the lateral ends of the hyoid extend as the cornua (C), 

 while hypobranchial and copular plates fuse. The adult (D) has a 

 large median plate, the hyoid body, bearing the long cornua which 

 are connected with the otic capsule, while behind are two posterior 

 cornua which contain the only ossifications, and which are said, in 

 Rana, not to be a part of the branchial arches, although this is 

 affirmed for some Anura. 



AMNIOTA must have had a common origin, but, as shown by 

 the large number of cranial bones, they cannot have come from any 



Fig. 144. — Chondrocranium of Lacerla (Gaupp, 'oo). aop, antorbital plate; 

 bpl, basipterygoid process; c, entrance to nasal conch; col, columella; fh, fenestra 

 hypophyseos; /^o, postoptic fenestra; na, nasal capsule; nf, notochord; of, optic foramen; 

 pa, posterior ampulla; pi, pterygoid; q, quadrate; Ic, trabecula communis; tmg, marginal 

 taenia; Ir, trabecula; VII, XII, nerve e.xits. 



existing order of Amphibia, but must have had a Stegocephalian 

 ancestry, which may have been the diverging point of Sauropsida 

 and MammaHa; or this separation may have occurred after the 

 appearance of the Theromorphs, there being evidence in favor of both 

 views. 



