ON THE VERTEBRATE SKELETON. 193 
midal-shaped petrosal of mammals and birds, makes its appearance between 
the alisphenoid, exoccipital and basioccipital, as at 16, fig. 9. Here, however, 
it is necessary to offer a few observations on the sense in which I use the 
térm ‘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- 
Jaginous in many fishes and saurians, and in all batrachians, ophidians and 
ehelonians, and is contained in a cavity or orbit (otocrane) which most, or all 
of the elements of the occipital and parietal vertebrae concur 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- 
eranial 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 laminz of the exoccipital, ali- 
sphenoid, and supraoccipital bones, a fourth osseous element (16), 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 harmonize 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 
cochlez’: 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 cochlez,’ and is con- 
tinued into a thin plate of bone which divides the ‘ fenestra cochlez’ 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 fenestre+. 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 4 trois branches, Cuvier, /. c. p. 165. 
Du cote de la caisse la paroi est percée de deux fenétres trausversalement oblongues et 
5 a p p gz 
séparées par un filet mince.” /. c. p. 82. 
