194 REPORT— 1846. 
Neither the figure of the interior surface of the cranium of the crocodile, 
which Spix gives as that of the Nilotic species in his great ‘Cephalogenesis,’ 
tab. ii. fig. 6; nor the figure given by Geoffroy of the skull of his Crocodilus 
suchus in the ‘ Annales des Sciences,’ tom. iii. pl. 16, fig. 2; nor that of the 
Crocodilus biporcatus, which illustrates the later memoir by the same author 
in the ‘Mémoires de l Académie Royale des Sciences,’ t. xii. (1833), pl. 1, 
fig. 2.; nor that (if it be an original figure) published by Dr. Hallmann in 
his ‘Comparative Anatomy of the Temporal Bone’ (taf. iii. fig. 49), give any 
indication of this, in the determination of the homology of the alisphenoid 
and petrosal, most significant and important ossicle. The proof of its normal 
character will be afforded by comparisons of the description and figure of 
the part here given with a section of the cranium of any true Crocodilus, 
Alligator or Gavial. In the latter, the otocranial plates of the alisphenoid, 
exoccipital and supra-occipital, project considerably into the cranial cavity. 
Any one of these plates might be called ‘ petrosal,’ for such reasons as have 
induced Cuvier to apply that name to the alisphenoid in the crocodile and 
other reptiles*. We find, indeed, that Geoffroy has applied the equivalent 
term, by turns, to each. But the true idea of the petrosal should include all 
those gristly and bony parts of the immediately investing capsule of the la- 
byrinth which occupy the otocranial excavations of the exoccipital, supraoc- 
cipital and alisphenoid ; and as the ossified portions of the true petrosal, in the 
crocodile, usually contract a bony union with the parietes of the otocrane, 
all these bony portions of the immediate capsule of the labyrinth might be 
called ‘petrosal processes’ of the bones to which they respectively adhere. 
That portion which unites to the exoccipital is attached by two lamelle; it 
forms a great part of the cochlear cavity, the lower half of the posterior semi- 
circular canal and the hinder half of the external or upper semicircular canals: 
that plate which belongs to the supra-occipital is attached to its otocranial 
surface by three points, and forms the upper third part of the anterior semi- 
circular canal and the crus of the posterior canal which communicates there- 
with : that part which adheres to the alisphenoid forms the anterior crus of the 
anterior (in Man superior) semicircular canal and the anterior beginning of the 
external canal. The proper and usually distinct bony portion of the petrosal 
(fig. 9, 16), which articulates with both alisphenoid and exoccipital, forms 
part of the ‘meatus internus,’ nearly the whole of the ‘ fenestra cochlez,’ and 
half of the ‘fenestra vestibuli’: it can only be regarded a ‘ petrosal process’ 
of the exoccipital by virtue of the very limited anchylosis occasionally con- 
tracted by the thin plate dividing the two ‘fenestre,’ along with the true 
petrosal process of the exoccipital above described. 
If we compare with 
the inner wall of the cro- 
codile’s cranium that of 
an ophidian, the python 
for example (fig. 10), we 
shall find the walls of the 
‘otocrane’ or chamber 
of the labyrinth to be 
contributed by the ex- 
occipital, (2) supra-oc- 
cipital(s )andalisphenoid 
(6) in nearly equal pro- 
portions ; the basioccipi- = 
tal (1), also, being ac- Cranium ofa python partially bisected. Natural size. 
* Ossemens Fossiles, 4to. 1824, y. ii. pp. 81, 180, 258. 
Fig. 10. 
