196 report — 1846. 



M. Agassiz is perfectly accurate in his character of the petrosal, according 

 to its relative position, as completely investing the entire labyrinth (of which, 

 by the way, the semicircular canals are an integrant part in all vertebrates 

 and form almost the whole in fishes) ; but he talces a narrow view of its 

 histological characters. The sclerotic is not less essentially a sclerotic in the 

 shark, where itis cartilaginous, than it is in the cod, where it is osseous; neither 

 is it less the eye-capsule and homotype of the petrosal in the mammal because 

 it retains the earliest histological condition of the skeleton, viz. that of a fibrous 

 membrane. And, in point of fact, in those fishes where the essential parts of 

 the internal organ of hearing appear to be protected solely by the parietes of 

 the bones, which, in the animals where the petrosal is ossified, or, as M. Agassiz 

 expresses the fact, ' exists,' surround such petrosal, the vascular and nervous 

 parts of the labyrinth are actually in such fishes more immediately enveloped 

 by the petrosal in its membranous or cartilaginous states. What is peculiar 

 to the petrosal in fishes is, that it is never entirely ossified ; and, furthermore, 

 that whenever itis partially ossified, the bony part is external and appears on 

 the outside of the skull instead of the inside, as in the crocodile and birds. 



In the chelonia, a larger proportion of the petrosal intervenes between the 

 alisphenoid and exoccipital upon the inner wall of the cranial cavity than in 

 the crocodile ; but it is wholly cartilaginous. In the bird, on the contrary, the 

 whole petrosal capsule of the organ of hearing soon ossifies and becomes 

 firmly anchylosed to the parts of the exoccipital, mastoid, alisphenoid and 

 basisphenoid that form its primitive chamber or otocrane ; owing, however, 

 to the larger relative size of the ossified part of the proper capsule (petrosal 

 proper) which penetrates the cranial cavity, none of the surrounding bones 

 which contribute accessory protection, have received the name of 'rocher,' 

 or pars petrosa. It is chiefly from not recognizing or appreciating the general 

 nature or homology of the ' petrosal ' that Cuvier failed to perceive its special 

 homology in reptiles. Speaking of the skull of the crocodile, he says that 

 the petrosal, or ' rocher,' is not less recognizable than the 'tympanic' and 

 other so-called dismemberments of the temporal by its internal position, 

 by its lodging a great part of the labyrinth, and by its contributing essen- 

 tially to the formation of one of the fenestrae (l. c. p. 81). But the part in 

 the crocodile which I regard as homologous with Cuvier's ' rocher ' in the 

 perch, is more completely internal in position than is Cuvier's so-called 

 ' rocher ' in the crocodile : it contributes a greater share to the formation 

 of the ' fenestra vestibuli,' and it forms almost the whole of the ' fenestra 

 cochlea?.' It is not true of the alisphenoid (Cuvier's l roc/ier ,s ) in the cro- 

 codile, that it lodges a great proportion of the labyrinth*: the otocranial 

 or petrosal process of the alisphenoid lodges a part only of the anterior 

 semicircular canal, and no part at all of the other semicircular canals. The 

 exoccipital is that tributary of the otocrane which lodges the major part 

 of the labyrinth ; it contains, for example, parts of two semicircular canals, 

 and the rudimental cochlea : and, when the middle, usually distinct part 

 of the petrosal is joined to it, the exoccipital may be said to form the 

 whole ' fenestra cochleae ' and a greater part of the ' fenestra vestibuli.' We 

 see, then, that the characters by which Cuvier deems his ' rocher' to be so 

 easily recognizable, are more prominent in the exoccipital than in the ali- 

 sphenoid : and the choice of the latter by Cuvier as the representative of 

 the ' rocher,' seems chiefly to have been influenced by the more obvious and 

 unmistakeable essential (neurapophysial) characters of the ' occipital lateral ' 

 (fig. 9, 2), whilst the accessory character which this bone derives from its 

 lodging and becoming confluent with part of the true petrosal, was not allowed 



* "II loge en graiide partie le labyriiithe," I. c. p. 81. 



