70 COMPAKATIYK ANATOMY. 



Anteriorly, the opercular apparatus lies against a bony chain 

 consisting of three pieces tlie hyomandibular, symplectic, and 

 quadrate which serves as a suspensorial apparatus for the lower 

 jaw (Fig. 55, Jim, s f . V". and Fig. 4S, D). The latter consists of 

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

 is called the deiitary l''i'.;. .">.">. lie}. The others are the articular 

 (Fig. 55, Ar), angular, and < onmoid. The last, howe\er. is as a 

 rule absent, and the angular may also he wanting. 



A curious asymmetry is seen in tin- head of adult Pleuroncct i(l;i\ AYhen 

 hatched, these Fishes are quite symmetrical, but later i.n the eye of one 



side lier. lines rotated, so that eventually lioili eyes come to lie on the same >ide : 

 in consequence of this, the skull also becomes asymmetrical. 



jj. 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 hones. 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. 50), in which the chondrocranium still plays a great 

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

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

 sules (Figs. ."><; to 5,s, oil-, 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. .">(' and 5S, 

 Fc}. 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 t\vo condyles for articulation with the lirst 

 vertebra are developed on the ventral periphery of the foramen 

 magnum (Figs. 56 to 5is, O'r). 



The large nasal capsules, consisting throughout life of consider- 

 able cartilaginous tracts (Fig. 57, No), are connected with the 

 auditory capsules by means of the t rabeculse, 2 which form tin- 

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

 cavity becomes closed dorsally by the f rentals and pariefals (Fig. 

 57, F, P], and ventrally by the parasphenoid (Figs. 5ii and 58, 

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

 of many Teleostci. In front of it lie the vomers (Figs. 5G and 

 5S, Vo), which bound the posk-rior nostrils, and in adults each 



1 Tin- re are novcr nmiv tlcm rudiments of a supra- and basioccipitaJ in Auura, 

 and not even rudiments of these lnuics in ('rodeles. 



-' The tralierii],r become more or less entirely o^Mied HN the s ]>li en et linio id and 



]H'oo1 ies. 



