MUSTELUS L.^VIS. 121 



of the canals in Scomber and G-adus^ but still other differences 

 here exist. In Scomber, that part of the infra-orbital line 

 that corresponds to the anterior, commissural section here in 

 question, has become fused (4) with the anterior end of the 

 supra-orbital canal, and forms a direct anterior prolongation of 

 that canal. In Gadus (12) the same section of canal forms 

 a short and direct anterior prolongation of the infra-orbital 

 canal, containing, as in Scomber, but a single sensory organ. 

 In both Scomber and Gadus the single nasal aperture of 

 the one, and the two apertures close together of the other, lie 

 relatively close to the eye, posterior to the anterior ends 

 of both the supra-orbital and antoi-bital canals, and there is 

 no indication of the loop, wholly or partly encircling the 

 nostrils, that is found in Mustelus, Amia, Polypterus, and 

 Batrachus. 



In other teleosts and ganoids the descriptions of this part 

 of the lateral canals are either not clear or not sufficiently 

 detailed or complete to warrant an attempt to establish 

 their homologies. This applies even to Herrick's (32) care- 

 ful work on Menidia, in which I am unable to trace the 

 relations of the several surface sensory organs he describes 

 in this region, either to each other or to the lateral canals, or 

 even to decide whether they are lateral sensory pit organs, or 

 terminal buds. 



In Liemargus (18) the only points in which the canals 

 here differ from those of Mustelus are that the anterior end 

 of the infra-orbital canal does not anastomose with the supra- 

 orbital, and that the distal, turned-back end of the supra- 

 orbital canal anastomoses with the infra-orbital canal at a 

 point that seems to lie morphologically posterior to the one 

 where the anastomosis takes place in Mustelus. 



In Raia (19) the two anterior sections of Ewart's descrip- 

 tions of the infra-orbital canal, sections 10^ and 10''', each form, 

 at their anterior end, a transverse anastomosis with the cor- 

 responding canal of the opposite side of the head. Between 

 these two anastomoses the canals of opposite sides lie parallel 

 to each other, and not far apart. These canals thus differ 



