The Latero-Sensory Canals and Related Bones in Fishes. 453 



ments could naturally only be homologous if the middle head line of 

 Salmo was homolog'ous with the supratemporal canal of Chimaera. 



The supraorbital canal traverses the nasal and frontal b,ones, and 

 anastomoses with the main infraorbital canal by what is apparently 

 its terminal tube, the anastomosis taking place at the anterior end 

 of the squamosal. 



The main infraorbital canal traverses the circumorbital bones, as 

 shown in the figure, and the dorsal one of the three bones usually 

 considered as the postorbital bones, is quite certainly the postfrontal 

 of the fish. As in Scomber [11], the infraorbital canal, as it passes 

 from the dorsal edge of this postfrontal bone to the anterior end of 

 the squamosal lies for a short distance in the dermis only. In Salmo 

 salar and Salmo farlo, Schleip [66] says that the anterior bone of the 

 suborbital chain is not travei'sed by the infraorbital canal, that canal 

 beginning in the second bone. This anterior bone, shown in Parker's 

 figure of Salmo salar, must be found also in Salmo namaycush, but 

 it is not shown in my sketches doubtless simply because it was not 

 traversed by the sensoiy canal. It would seem to be developed in 

 relation to the hind end of the ethmoidal line of pit organs, and hence 

 to be the antorbital bone of the fish. 



The sphenotic (postorbital ossification) does not, in Salmo namaycush, 

 come to the outer surface of the skull, as Parker shows it in Salmo 

 salar. Its dorsal surface even seems, in my drawings, to lie directly 

 internal to, and to give support to a projecting corner of the frontal. 



A dermal, latero-sensory component of the squamosal is shown, 

 apparently quite easily separated from an underlying auto-squamosal 

 or true pterotic. 



The extrascapular is represented, in the two sketches that I have, 

 by one lateral ossicle and two or more mesial ones. The lateral 

 ossicle encloses a section of the main infraorbital canal, and doubtless 

 lodges a single sense organ of that line. The mesial ossicles enclose 

 the supratemporal canal, the canals of either side anastomosing in the 

 lid-dorsal line. In one of my sketches, said to be of an unusually 

 large specimen, there are four of these mesial ossicles on each side. 

 [In the other, which only shows the canal of one side, there are two 



