ON THE VERTEBRATE SKELETON. 193 



midal-shaped petrosal of mammals and birds, makes its appearance between 

 the alisphenoid, exoccipital and basioccipital, as at i6, fig. 9. Here, however, 

 it is necessary to offer a few observations on the sense in which I use the 

 term ' petrosal ' as applied to that ossicle. 



The petrosal, properly so called, considered in its totality, as the immediately 

 investing capsule of the labyrinth or internal organ of hearing, is wholly carti- 

 laginous in many fishes and saurians, and in all batrachians, ophidians and 

 chelonians, and is contained in a cavity or orbit (otocrane) which most, or all 

 of the elements of the occipital and parietal vertebrae conci^r in forming. A 

 part of the ear-capsule remains cartilaginous in the crocodile; but several 

 portions become ossified around the semicircular canals and rudimental 

 cochlea, which ossifications contract slender adhesions to the smooth oto- 

 cranial surfaces of the supraoccipital, exoccipital and alisphenoid ; and to 

 one of these portions (on the principle on which Cuvier applies the term 

 ' rocher' in fishes) the name petrosal might more particularly be given, as it 

 is more distinct and moveable than the other partial ossifications of the cap- 

 sule, and contributes to form the ' meatus internus ' towards the cranial cavity, 

 surrounds nearly the whole of the ' fenestra rotunda', and one-half of the ' fe- 

 nestra ovalis' towards the tympanic cavity. Looking upon the inner surface 

 of the lateral walls of the cranium (as at fig. 9), one sees at the bottom of 

 the T-shaped suture* uniting the otocranial laminae of the exoccipital, ali- 

 sphenoid, and supraoccipital bones, a fourth osseous element (ic), presenting 

 a convex extremity towards the cranial cavity, and completing, with the exocci- 

 pital, the lower half of the foramen for the nervus vagus. If this little bone 

 be pressed upon with a needle or probe, it yields and moves, being divided 

 by smooth harmonise from both the exoccipital (2) and alisphenoid (6). 



The protuberance in question, which thus projects into the cranial cavity, 

 is the rounded angle of the border of the inferior plate of the petrosal, which 

 joins the exoccipital. This lower horizontal plate of the petrosal forms the 

 upper wall of the ' fissura lacera posterior,' and the lower wall of the ' fenestra 

 cochleae': the fore-part of the horizontal plate bends upwards, twisting 

 and expanding into a vertical oval plate, articulated by its anterior surface to 

 a corresponding sutural surface of the alisphenoid. The lower margin of 

 this plate forms the upper boundary of the ' fenestra cochleae,' and is con- 

 tinued into a thin plate of bone which divides the ' fenestra cochleae' from the 

 ' fenestra vestibuli ' above. This thin plate of the petrosal joins and is usually 

 anchylosed to the exoccipital: it is the only part of the true petrosal noticed 

 by Cuvier, who describes it as a slender filament of bone which separates 

 the two fenestraef. Seen edgewise, looking into the tympanic cavity, the 

 plate appears like a filament : and this plate forms the sole connection, when 

 any exists, between the petrosal and the exoccipital. I have always found 

 the sutures persistent between the petrosal and the alisphenoid. The upper 

 border of the 'fenestra vestibuli' is formed by a petrosal, or rather otocra- 

 nial, process of the alisphenoid. 



The part (fig. 9, 16) entering into the formation of the lateral walls of the 

 brain-case, and which is here specially indicated by the name of ' petrosal,' 

 seems to have been overlooked : it is, however, relatively to the alisphenoid 

 or exoccipital, as large as is the petrosal (Cuvier's rocher) in the perch : it 

 has a true osseous texture, and is quite distinct from the lenticular mass of 

 calcareous matter in the adjacent cochlear chamber which Cuvier compares 

 to starch (' amidon durci '). 



* Suture a trois branches, Cuvier, I. c. p. 1G5. 



f Du cote de la caisse la paroi est percee de deux fenctres transversalement oblongues et 

 sepavees par un filet mince." /. c. p. 82. 



