120 EDWARD PHKLPS ALLIS, JUN. 



of the canal that I cousidered as an anterior section or 

 commissure of the line. In Amia there are two nasal 

 apertures on each side of the head. Between these two 

 apertures the backwardly-directed distal end of the supra- 

 orbital canal is dii-ected toward and approaches somewhat 

 the hind end of the so-called anterior, commissural section 

 of the infra-orbital canal. If two separate nasal apertures 

 had been developed in Mustelus, and the posterior one had 

 travelled backward toward the anterior edge of the eye, 

 it would necessarily, since the nose develops earlier than the 

 lateral canals, have passed backward between the growing 

 distal end of the supra-orbital canal and the point where 

 that canal is later to anastomose with the infra-orbital canal. 

 This would thus have here given closely the arrangement 

 found in Amia. It would also probably closely give the 

 arrangement found in Polypterus (6), but I have not yet 

 worked out the details of the innervation in that fish. In 

 Batrachus tau a very similar arrangement is also found 

 (10), but in this latter fish the anterior organ of the supra- 

 orbital line is not enclosed in, nor does it lie in the line 

 of, the antero-laterally directed distal end of the supra-orbital 

 canal. 



In the projection of the canals of Mustelus the anterior end 

 of the infra-orbital canal anastomoses with the supra-orbital 

 canal at or near the point where that canal bends sharply 

 backward, postero-mesial to the nasal aperture. It then 

 joins and anastomoses with its fellow of the opposite side, and 

 here forms a short, median, longitudinal section, which lies on 

 the an tero- dorsal surface of the snout, between the nasal 

 apertures of opposite sides of the head. In both Amia and 

 Polypterus the arrangement is here quite different, and there 

 are also differences in the arrangements presented by these two 

 fishes. They can, however, all be easily derived one from 

 the other, and in Conger conger I have lately found exactly 

 the arrangement here shown in the projection of Mustelus. 

 There is thus certainly a full homology in this part of the 

 lateral system of these several fishes. The same is also true 



