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 takes 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 ofa 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. Itis 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 fenestre (/. 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 
cochlee.’ It is not true of the alisphenoid (Cuvier’s ‘rocher’) in the ecro- 
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 cochlez’ 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 latéral’ 
(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 
* “T) loge en grande partie le labyrinthe,” /. c. p. 81. 
