84 



COMPARATIVE ANATOMY 



regions, has very distinctly the relations described in the introduc- 

 tion to this chapter. The auditory capsules (Figs. 63 to 65) — which 

 are bound together by cartilaginous tracts in the basi- and supra- 

 occipital regions, and generally become strongly ossified later by 

 the exoccipitals and prootics, — show a new and important 

 modification as compared with those of Fishes in the presence of 

 an aperture, the fenestra ovalis, on the outer and lower side 

 of each. This fenestra is closed by a cartilaginous plug, the 

 stapedial plate, probably corresponding to a part of the wall of 

 the auditory capsule ; from it a rod-like cartilage or bone, the 

 columella auris, corresponding phylogenetically to the upper 

 element of the hyoid arch, extends outwards towards the quadrate 

 in most Urodeles and serves to conduct the sound to the inner 

 ear, the position of the semicircular canals of which is indicated 

 by corresponding cartilaginous ridges on the capsule. 



In all Amphibians two condyles for articulation with the first 

 vertebra are developed on the ventral periphery of the foramen 



Fig. 66. — Skull and Visceral Aeches of Menopoma. (From the side.) 



I, mandible ; II, hyoid ; III-VI, branchial arches ; qn, quadrate, above wliioh 

 is the squamosal ; ar, articular ; mi:, Meckel's cartilage, enclosed by the 

 dentary bone. 



magnum. The occipital region is ossified by two exoccipitals, a 

 bony supra- and basioccipital rarely being present in recent 

 forms (certain Anura). 



The large nasal capsules, consisting throughout life of consider- 

 able cartilaginous portions, are connected with the auditory 

 capsules by means of the trabecultE, which give rise to the side 

 walls of the skull and become more or less entirely ossified as 

 the sphenethmoid and prootics. The cranial cavity is closed 

 dorsally by the frontals and parietals, and ventrally by the 

 parasphenoid, which is sometimes provided Avith teeth. In 

 front of it are the vomers, which bound the internal nostrils ; in 

 adults each vomer becomes fused with the corresponding palatine, 

 which forms a delicate bar lying on the ventral surface of the 



