70 COMPARATIVE ANATOMY. 



Anteriorly, the opercular apparatus lies against a bony chain 

 consisting of three pieces the hyomandibular, sympiectic, and 

 quadrate which serves as a suspen serial apparatus for the lower 

 jaw (Fig. 55, Hm, S, Qu, and Fig. 48, D). The latter consists of 

 Meckel's cartilage and of several bony elements, the largest of which 

 is called the dentary (Fig. 55, Be). The others are the articular 

 (Fig. 55, Ar), angular, and coronoid. The last, however, is as a 

 rule absent, and the angular may also be wanting. 



A curious asymmetry is seen in the head of adult Pletironec.tidae. When 

 hatched, these Fishes are quite symmetrical, but later on the eye of one 

 side becomes rotated, so that eventually both eyes come to lie on the same side ; 

 in consequence of this, the skull also becomes asymmetrical. 



B. Amphibia. 



Urodela. The skull of tailed Amphibians is distinguished 

 from that of Fishes principally by negative characters, on the one 

 hand by the presence of less cartilage in the adult, and on the 

 other by a reduction in the number of bones. In short, it presents 

 altogether a much simpler plan, reminding us of that of Ganoids 

 and Elasmobranchs. This is seen, for instance, in the larval con- 

 dition (Fig. 56), in which the chondrocranium still plays a great 

 part, its auditory, nasal, arid orbital regions having the relations 

 described in the introduction to this chapter. The auditory cap- 

 sules (Figs. 56 to 58, OB], which are bound together by cartila- 

 ginous basi- and supraoccipital tracts, 1 and generally become strongly 

 ossified later, show a new and important arrangement as compared 

 with those of Fishes in the presence of an aperture, the fenestra 

 ovalis, which looks outwards and downwards (Figs. 56 and 58, 

 Fov). This fenestra is closed by a cartilaginous or bony plug, the 

 stapes, and will be spoken of again in connection with the anatomy 

 of the auditory organ. 



In all Amphibians two condyles for articulation with the first 

 vertebra are developed on the ventral periphery of the foramen 

 magnum (Figs. 56 to 58, Cocc). 



The large nasal capsules, consisting throughout life of consider- 

 able cartilaginous tracts (Fig. 57, Na\ are connected with the 

 \) auditory capsules by means of the trabeculse, 2 which form the 

 side walls of the skull, and enclose a large cavity. This cranial 

 cavity becomes closed dorsally by the frontals and parietals (Fig. 

 57, F } P), and ventrally by the parasphenoid (Figs. 56 and 58, 

 Ps), which is sometimes provided with teeth similar to those 

 of many Teleostei. In front of it lie the jgMMps (Figs. 56 and 



58, Vo), which bound the posterior nostriM^K in adults each 



1 There are never more than rudiments of a supra- and basioccipital in Anura, 

 and not even rudiments of thes,> bones in Urodeles. 



2 The trabecula 1 become more or less entirely ossified as the sphencthmoid and 

 prootics.. 



