2 12 FISHES CHAP. 
it may be stated that there are formed in that part of the 
parachordal cartilage surrounding the foramen magnum a median 
basioccipital below, which is concave behind where it articulates 
with the centrum of the first vertebra, a swpraoccipital above, 
and two laterally-placed ewoceipital bones (Figs. 121, 122). 
Each periotic capsule is ossified by the formation of five bones 
in the primitively cartilaginous mass, the prootic, sphenotic, opisth- 
otic, epiotic, and the pterotic. The inner walls of the capsules 
have atrophied in the adult, and hence the cavities which contain 
the auditory organs appear as open lateral recesses of the cranial 
cavity. In front of the periotic capsules there are various bones 
which are formed in the cartilage of the trabecular part of the 
Q ooo --9------- 
th 
§ 
Fic. 121.—Side view of the cranium of a Salmon (Salmo salar). Most of the membrane 
bones and the jaws have been removed. The cartilage is dotted. a.s, Alisphenoid; 
bo, basioccipital; 6s, basisphenoid ; eo, exoccipital; ep, epiotic; J.eth, lateral 
ethmoid ; o/, olfactory capsula; op, opisthotic ; 0.s, orbito-sphenoid ; pr.o, pro- 
otic ; ps, parasphenoid ; pt.o, pterotic ; so, supraoccipital ; sp.o, sphenotic ; ¢.¢, 
trabecular cornu; w./.c, u.l.c¥, first and second upper labial cartilages ; v, vomer ; 
II, foramen for the optic nerve. (From W. K. Parker.) 
cranium. Thus, in front of the basi-occipital, and developed in 
the cartilage of the cranial floor, there is a median Y-shaped basi- 
sphenoid, and, at some distance above it on each side, an ali- 
sphenoid helps to form the lateral wall of the cranial cavity. 
Between the eyes the side walls of the cranium fuse to form a 
vertical inter-orbital septum, and, in consequence, two orbito- 
sphenoid bones, which normally form the lateral cranial walls in 
this region, become partially confluent in the median line and 
close the cranial cavity in front. The only cartilage bones 
found in the massive persistent portion of the primary cranium 
which forms the pre-orbital region are the projecting lateral 
ethmoids, forming the posterior boundaries of the recesses for the 
olfactory organs, and separating the latter from the orbits. 
The roof and floor of the primary cranium is completed by 
