THE SKULL OF THE SALMON. 89 



ridge. It is separated by two tracts of unossified cartilage 

 from the large series of bones connected with the auditory 

 organ. The first of these is the epi-otic (fig. 9, 2), which is 

 separated by only a narrow tract of cartilage from the supra- 

 occipital, and is continuous laterally with the large pterotic 

 (fig. 9, A, 3) which overlaps in front a smaller bone, the 

 sphenotic (fig. 9, 4). Both epi-otic and pterotic are drawn out 

 into rather prominent backwardly-projecting processes. 



The greater part of the remainder of the dorsal surface 

 is formed of unossified cartilage which is pierced by three 

 large vacuities or fontanelles. The anterior fontanelle is 

 unpaired, and lies far forward near the anterior end of the 

 long cartilaginous snout, the two larger posterior ones lie 

 just in front of the supra-occipital and lead into the cranial 

 cavity. In front of the orbit the skull widens again, and 

 is marked by two considerable lateral ethmoid (fig. 9, 8) 

 ossifications. In front of these are a pair of deep pits, the 

 nasal fossae, at the base of which are a pair of foramina 

 through which the olfactory nerves pass out ; they communicate 

 with a space, the middle narial cavity, seen in a longitu- 

 dinal section of the skull. 



The long cartilaginous snout is more or less bifid in front, 

 especially in the male (fig. 9). 



POSTERIOR END OF THE CRANIUM. 



The foramen magnum forms a large round hole leading 

 into the cranial cavity, and is bounded laterally by the two 

 exoccipitals and below by them, and to a very slight extent 

 by the basi-occipital, the three bones together forming a 

 concave occipital condyle by which the vertebral column 

 articulates with the skull. 



The exoccipitals are connected laterally with a fourth 

 pair of auditory bones, the opisthotics, and just meet the 

 epi-otics dorsolaterally, while dorsally they are separated by a 

 wide tract of unossified cartilage from the supra-occipital. 



The opisthotics are connected laterally with the pterotics. 



