The Latero-Sensory Canals and Related Bones in Fishes. 483 



mesially through that bone it then, in my large specimen, entered and 

 traversed a second extrascapular bone, on each side of the head, these 

 two bones not here having fused to form a single median bone, such 

 as is shown by Parker, Huxley and Gegenbaur. No record was kept 

 as to whether there were, here, in the smaller specimen, one or two 

 bones. In this smaller specimen a group of two or more sense organs 

 was found, on each side, in that part of the canal that lies in the 

 lateral extrascapular, but no nerve could be traced, on either side, to 

 the median extrascapular. This latter bone may accordingly not be 

 a latero-sensory skeletal unit. The lateral extrascapular organs were 

 innervated by branches of the nerve that innervates also the extra- 

 scapular organ of the main infraorbital canal. 



The supraorbital canal begins at the antero-ventral edge of the 

 nasal pit, and from there runs upward and backward in the roof of 

 the pit, passing between the nasal apertures. In this part of its 

 course it traversed, in the one specimen examined, three small tubular 

 bones on one side of the head, and four on the other. Beyond the 

 dorso-posterior edge of the nasal pit the canal continues backward in 

 a curved, circumorbital course, traversing first the nasal bone of both 

 Huxley's and Gegenbaur's descriptions and then the frontal bone. 

 Beyond this latter bone it enters the squamosal and there soon ana- 

 stomoses with the main infraorbital canal. The three or four little bones 

 that lie in the roof of the nasal pit, probably each lodge at least one 

 sense organ of the line, and these organs are probably all innervated 

 by the ramus ophthalmicus facialis, but no branch of that nerve could 

 be traced to the most anterior bone. These little bones thus cer- 

 tainly form part, if not all, of the true nasal of the fish, the so-called 

 nasal of earlier descriptions, which also lodges one or more sense or- 

 gans, innervated also by the ophthalmicus, perhaps also forming part 

 of that bone; though it would seem to be a bone which, like the so- 

 called ethmo-nasal of Lepidosteus, can be considered as a part of the 

 frontal with as much or even more reason than as a part of the 

 nasal. Collinge [29] says that the supraorbital canal of Acipenser 

 continues forward beyond the nasal apertures, along the dorso-lateral 



31* 



