THE OSTEOLOGY OF AMIURUS CATUS. 275 
parasphenoid lies below the basioccipital behind, and also passing 
forwards, the prootic, basisphenoid, orbitosphenoid and ectethmoid, 
9. BASISPHENOID. 
Does not appear as a distinct bone in the skull of Amiwrus but is 
anchylosed with the subjacent parasphenoid, the line of demarcation 
between the two being more or less distinct however. It is a flat- 
tened impair bone, presenting no especial features for examination. 
It forms the lower boundary of the foramen for trigeminus and 
facialis behind, and partly of the foramen for opticus in front, and 
articulates behind with the prootic ; externally with the alisphenotd ; 
in front with the orbitosphenoid ; and below with the parasphenord. 
10. AuispHenorp, (Pl. II. Fig. 2, AS.) 
A rather small bone, lying on either side between the foramina, of 
which it forms the anterior and posterior boundaries, respectively. 
These foramina are that for the trigeminus and facial behind, and 
that for the opticus in front. The bone is very roughened and 
ridged on its external face for the attachment of muscles, and above 
this roughened portion is a hollow in which lies the anterior portion 
of the hyomandibular. The inner surface is smooth. From the pos- 
terior edge a spicule of bone passes backwards in those individuals, 
in which the foramen for passage of the 5th and 7th nerves is divided 
completely, which spicule unites with a similar one from the prootic. 
Immediately in front of posterior edge is a small foramen for exit of 
the ciliary trunk of the 5th nerve. On the inner side, immediately 
above the inferior process, which articulates with the basisphenoid, 
are two foramina, one above the other. The inferior of these is the 
larger, and opens into a canal, pursuing a course more or less oblique 
in different individuals to the exterior. It gives passage to the deep 
branch of &. ophthalmicus trigemini. The smaller one lies at the 
extremity of a longitudinal groove, and opens into the interior of the 
bone like other similar foramina which perhaps, have a nutritive 
function. Hach alisphenoid articulates above with the sphenotic 
and frontal ; behind with the prootic ; below with the basisphenoid; 
and in front with the orbitosphenoid. 
11. Fronvats, (Pl. II. Figs 1 & 2, Fr.) 
Are flat plate-like bones, with a small ridge projecting downwards 
from the middle of the under surface. They are separated from each 
