212 



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 supraoccipital above, 

 and two laterally-placed exoccipital bones (Figs. 121, 122). 

 Each periotic capsule is ossified by the formation of five bones 

 in the primitively cartilaginous mass, the prootic, sphenotic, opistli- 

 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 



ol 



u/.l. 



FIG. 121. Side view of the cranium of a Salmon (Salmo solar). Most of the membrane 

 bones and the jaws have been removed. The cartilage is dotted, al.s, Alisphenoid ; 

 bo, basioccipital ; bs, basisphenoid ; eo, exoccipital ; ep, epiotic ; l.eth, lateral 

 ethmoid ; ol, olfactory capsula ; op, opisthotic ; o.s, orbito-sphenoid ; pr.o, pro- 

 otic ; ps, parasphenoid ; pt.o, pterotic ; so, supraoccipital ; sp.o, sphenotic ; t.c, 

 trabecular cornu ; u.l.c, u.l.c' 2 , 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 



