_— c 
qe 
THE OSTEOLOGY OF AMIURUS CATUS. 27 
zontal portion of the supraoccipital, are parts of the same cavity. 
In a skull from which all accessory parts have been removed, it 
opens by a comparatively wide opening at the base of the ridge, 
which extends upwards upon the bone to unite with the similar 
ridge on the supraoccipital spine. This opening is almost closed in 
the natural condition by the supraclavicle, a small opening only 
being left. The cavity is apparently quite shut off from any com- 
munication with the brain-cavity, and contains only fatty tissue. 
On the upper surface of the pterotic, on the projecting posterior 
portion, are several foramina—the openings of a .mucous canal, 
which passes forwards in an osseous canal, running along the outer 
edge of the bone. The smooth surface formed by pterotic, exoc- 
cipitals and epiotic lodges the utriculus. The pterotic articulates with 
the supraoccipital above ; the epiotics, and supraclavicular behind ; 
the exoccipitals, and prootics below ; and in front with the sphenotic. 
6. Proorics, (Pl. II. Fig. 2, Pr0.) 
Lie on each side immediately in front of the exoccipitals. Each 
is a somewhat quadrate bone, extending to the middle line 
below, where it articulates with the fellow of the opposite side, 
thus entering into the formation of the base as well as the walls 
of the skull. The middle portion of its inner surface is crossed 
by a ridge, notched outwardly, in which notch the anterior or 
sagittal semi-circular canal passes to the vecessus utriculi. Near 
the posterior edge is another smaller ridge, round the outer 
extremity of which the same canal turns in passing forwards from 
the wiriculus. Between these two ridges is a smooth hollow, with 
a very thin wall, which lodges the recessus utriculi. Below the 
prootics, where they meet in the middle line below and between them 
and the anterior portion of the basioccipital above, and the parasphe- 
noid below, is a small cavity. This is the almost aborted rudiment 
of the canal for the orbital muscles, which is largely developed in 
many fishes, but absent or very rudimentary in Silurus, Amiwrus, 
Gadus, Lophius, &e. The middle of the anterior edge of the prootic 
is notched variously in different individuals, sometimes possessing a 
single notch, at other times there being two more or less separated 
by an intervening osseous spicule. These notches are closed in front 
by the posterior edge of the alisphenoid, and through the foramina 
thus formed the fifth and seventh cranial nerves (trigeminus and 
