82 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



AMPHIBIA. Several points distinguish the amphibian from other skulls. The 

 chondrocranium is platybasic (p. 61) ; except for a small synotic tectum frequently 

 present, it is not roofed by cartilage; the otic capsule has a fenestra vestibuli occu- 

 pied by the stapes, a development connected with the power of hearing (p. 73); 

 there are two occipital condyles; and the quadrate is immovably united to the 

 cranium by two processes, an otic process, joining the otic capsule, and an 'ascend- 

 ing process' which reaches the upper margin of the trabecula, and which, in many 

 reptiles, often ossifies as the epipterygoid bone. 



FIG. 82. Chondrocranium of Amphiuma, lateral and dorsal views, aop, antorbital 

 process; ap, ascending process of quadrate (epipterygoid) ; ct, cornua trabeculae; de, 

 foramen, for ductusendolymphaticus;e/>, ethmoid plate; fo, fenestra vestibuli; m, Meckel's 

 cartilage; n, notochord; oc, olfactory capsule; ov, occipital vertebras; p, parachordal; q, 

 quadrate; s, stapes; t, trabecula; 2-8, nerve exists. 



The cartilage cranial bones are few. Usually only exoccipitals are developed 

 in the hinder region, while the rule is a single petrosal (prootic), but occasionally 

 epi-, opisth-, and pterotic occur. There is but a single pterygoid, while basi-, pre-, 

 and alisphenoids are not ossified. The membrane bones in existing amphibians 

 have separated from the integument and have sunk to a deeper position than in 

 fishes, but in the stegocephals the presence of grooves for the lateral line system 

 would indicate a close connexion between skin and bones. In the latter group the 

 membrane bones are numerous, but in existing species they are noticeably reduced. 

 Except in stegocephals and the caecilians there are large vacuities in both floor and 

 roof of the skull. The lower jaw also has a reduced number of bones, there being 

 at most five including the articulare and the mento-Meckelian. 



The most primitive conditions occur in the stegocephals, where, as the name 



