102 



LECTURE V. 



functions are less specialised, less confined to the particular organ 

 ultimately destined for their performance in the lower than in the 

 higher classes, we find in Fishes several bones taking part with the 

 special acoustic capsule in the lodgment of the labyrinth ; and it is 

 only in the higher Vertebrata that the capsule, under the name of 

 the ' petrous bone,' entirely and exclusively envelopes the labyrinth. 

 Its ossification commences later than that of the cranial neurapo- 

 pliyses, in the series of Osseous Fishes : there are species (e. g. Pike) 

 in which, after the ex-occipitals, ali-sphenoids, and orbito-sphenoids 

 have received their destined amount of ossification, the petrosal 

 still remains in the cartilaginous state : it is very small, yet never- 

 theless exists in the Carp (/%r, 35. 16) and Bream, where Cuvier and 

 Bojanus* describe it as a dismemberment of the mastoid: in the 

 Perch, however, where the petrosal is a little better developed than 

 in the Carp, Cuvier recognises its true homology : it is somewhat 

 larger in the flat-fish (e. g. Holibut), and in the Cod tribe attains 

 an equal size with the ali-sphenoid, which it resembles in form, 

 except that the notched margin is posterior (yjig. 30. 16). Here it 

 forms the posterior lateral wall of the cranium; articulates below with 

 the basi-occipital and basi-sphenoid, above with the mastoid and par- 

 occipital, behind with the ex-occipital, and before with the ali- 

 sphenoid: it supports the cochlear division of the labyrinth con- 

 taining the otolites. The cavities (otocranes) lodging the petrosals 

 and organs of hearing are completely separated from each other, and 

 are formed, on each side, by the ex-occipital, par-occipital, ali- 

 sphenoid, mastoid, and post-frontal : they are sometimes closed ex- 

 ternally, bvit open widely into the cranial cavity. 



Cranium of a Carp. 



The optic capsule, or sclerotic, {Jig. 30. 17) like the acoustic cap- 

 sule, is cartilaginous in all Chondropterygians, and also in the semi- 

 osseous fishes, as the Lepidosiren, the Lophius, the Lophobranchs 

 and Plectognathes. In most osseous fishes it is bony, and conunonly 



* xxxvi. p. .TOl. tab. 7. figs. 1. and .5. IG. 



