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 Geoftroy of the skull of his Crocodilus 

 suchus in the ' Annales des Sciences,' torn. iii. pi. 16, fig. 2; nor that of the 

 Crocodilus biporcatus, which illustrates the later memoir by the same author 

 in the 'Memoires de l'Academie Royale des Sciences,' t. xii. (1833), pi. 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 thi$, 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 lamellae ; 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 beginningof 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 cochleae,' 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 ' fenestra?,' along with the true 

 petrosal process of the exoccipital above described. 



If we compare with 

 the inner wall of the cro- ^'S* ^' 



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^ )and alisphenoid 

 (6) in nearly equal pro- 

 portions ; the basioccipi- 



tal \l), alSO, being EC- Cranium of a python partially bisected. Natural size. 



* Ossemens Fossiles, 4to. 1824, v. ii. pp. 81, 180, 258. 



