528 H. H. SWINNERTON. 



Lej^idosteus^ and hence probably in all fishes, is thus an 

 intercranial space opened secondainly to the orbits" (97, p. 10). 

 This can apply, however, only to the anterior part of this 

 canal in Teleosts. In Amia this canal is apparently com- 

 pletely closed behind and beneath by cartilage. In Alepo- 

 cephalus only the anterior region is thus closed, whilst the 

 hinder is, as in the stickleback, closed by the parasphenoid. 

 In larval Amia this canal is not present, but there is a well- 

 marked interparachordal fossa to which the eye muscles bear 

 the same relation as in the stickleback. It is probable, 

 therefore, that in this fish also a process of depression and 

 secondary growth goes on on either side of the fossa and 

 below the pro-otic ; but that, whereas in the other types the 

 fossa persists and transmits the eye muscles back again out 

 of the cranial cavity beneath the basis cranii, in Amia it 

 disappears, owing to continued cartilaginous growth. As far 

 back as the so-called pro-otic bridge, these muscles may be 

 said to run in an actual derivative of the cranial cavity ; 

 behind that they run in an extra-cranial space secondarily 

 enclosed. 



This interparachordal fossa is a common feature among 

 Teleosts. Sagemehl (85, p. 6Q) speaks of it as the space 

 between the pro-otics, and expresses doubt as to the homology 

 of this with the pituitary fenestra of other vertebrates. 

 Development justifies his doubt, by showing that the fossa is 

 related to the parachordals and the fenestra to the hinder 

 ends of the trabeculae. 



Though the Teleostean pro-otic bears the same relations to 

 the seventh nerve and the auditory organs as in the higher 

 vertebrates, those of its other parts which meet in the middle 

 line behind the interparachordal fossa have trespassed upon 

 the area occupied by the reptilian basisphenoid (Howes and 

 Swinnerton, 01, pi. iv, fig. 6). The interest of this is increased 

 by the fact that among reptiles there are traces of a paired 

 origin for the basisphenoid (ibid., p. 42). When allowance is 

 made for the fact that, owing to the union of the trabeculae 

 with the anterior end of the paiachordals in Teleosts, and 



