THE SKULL OF THE SALMON. 91 



the pro-otic is the sphenotic. The pterotic is marked by a 

 prominent groove often lined by cartilage, which is continued 

 forwards along a tract of cartilage between the pro-otic and 

 sphenotic. With this groove the hyomandibular articulates. 



There are considerable ossifications in the sphenoidal region 

 of the side of the cranium. The anterior boundary of the pos- 

 terior fontanelle is formed by the large alisphenoid, which 

 is continuous behind with the pro-otic and sphenotic, and 

 below with a slender basisphenoid. Both in front of and 

 behind the basisphenoid there are considerable vacuities in the 

 walls of the cranium ; through the posterior of these openings 

 (fig. 10, V.) the main part of the trigeminal nerve passes out, 

 and through the anterior one, the optic (fig. 10, II.). The 

 alisphenoid is continuous in front with the orbitosphenoid 

 (fig. 10, 10), which is pierced by the foramen for the exit of 

 the first nerve (fig. 10, I.), and in front of the orbitosphenoid 

 there is a large vacuity. The lateral ethmoid is seen in the 

 side view as well as in the dorsal view. Further forwards are 

 seen the olfactory pits, and the long cartilaginous snout. 



A ventral view of the cartilaginous cranium shows much 

 the same points as the side view. The basisphenoid appears 

 on the surface immediately in front of the basi-occipital. 



THE SKULL WITH MEMBRANE BONES. 



The dorsal surface. The greater part of the dorsal 

 surface in front of the supra-occipital is overlaid by a pair 

 of large rough f rentals (figs. 9, A, 5, and 10, 5). They cover 

 the posterior fontanelles and stretch over from the sphenotic 

 to the lateral ethmoid, forming a roof for the orbit. They 

 meet in the middle line behind, but in front are separated 

 by a narrow tract of unossified cartilage, and are overlapped 

 by the median ethmoid (figs. 9, A> 6, and 11, 6). At the sides 

 of the supra occipital behind the f rentals are a pair of small 

 parietals (tigs. 9, A, 7, and 11, 7). 



In a ventral view the cranium is seen to be chiefly 



