MOUPFTOLOOY OF TELROSTEAN HEAD SKELETON. 527 



The pro-otic (o. j^r.) has now extended to the borders of the 

 interparachordal fossa, and has met its fellow in the middle 

 line for some distance behind that. In the pike, salmon, and 

 Alepocephaliis it completely surrounds the exit of the fifth 

 nerve, but here that part which lies in front of this foramen 

 is suppressed, and is functionally replaced by overlapping 

 processes of the sphenotic, frontal, and parasphenoid. 



The process of depression of those parts bounding the 

 interparachordal fossa laterally has continued, so that this 

 region now appears to be a mere downward process of the pro- 

 otic (fig. 20, o.j;?'."), with its cartilaginous extremity mortised 

 into the sides of the parasphenoid (fig. 35, jxf., o. pr/'). This 

 appearance is enhanced by the fact that posteriorly each 

 process is continued into a ridge running along the under 

 surface of the hinder portion of the pro-otic (fig. 20). These 

 two ridges are continuous with those already described under 

 the basioccipital, and there is a channel thus formed which 

 runs a considerable length of the basis cranii, is closed 

 ventrally by the parasphenoid, and opens anteriorly into the 

 cavum cranii by means of the interparachordal fossa. In the 

 salmon this canal is much more strongly developed, and 

 opens posteriorly. Klein (84, p. 135) gives a list of those 

 families with and those without an eye-muscle canal, but he 

 errs in including Gasterosteus among the latter. A study of 

 his lists and of many of the original skulls themselves strongly 

 suggests that the degree of development of this feature is not 

 of fundamental importance, but is largely dependent on the 

 shape of the head — on its depth or its depression. 



A similar canal is described by Bridge and Sagemehl, and 

 with greater detail by AUis in Amia. Sagemehl regarded it 

 as a portion of the cavum cranii, which has become roofed 

 over by two horizontal lamellas from the pro-otic (84, p. 207). 

 The same idea is conveyed by him in regard to this feature 

 in Cyprinida3 and Characinidas, and also by Gegenbaur in 

 Alepocephalus. Each of these authors describes the rooL 

 as a bridge formed by lamelljB projecting from the pro-otic. 

 According to AUis, " the eye-muscle canal in Amia and 



