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Show 1895.] SENSOEY C A N A L S Y S T E M O F FISHES. 293 anterior portion it makes a slight dorsal curve and terminates on the region of the head by opening a wide saccular dilatation. The pores distributed along tbe surface of the canal are numerous, averaging in the posterior half of the body 7 to every 2 cm. 2. T H E M A I N C A N A L of the Head.-As previously mentioned, at the point where the lateral canal terminates there is a wide saccular dilatation, out of which the main canal opens. As soon as this dilatation has been cut open the entrance to the main canal appears as a definite circular opening. Its border is surrounded by a ring of cartilage, and the rest of the canal by a series of cartilages similar to that figured on PI. X X I . figs. 18,19. In very large and old specimens this cartilage becomes partially ossified. The canal passes inwards and downwards and through the frontal bone, returning to the surface again slightly posterior to the orbit, and divides into the supra- and sub-orbital branches. There are no other branches passing off from the main canal, and no pores opening to it from the surface of the head. The diameter of the canal is very irregular; in the posterior portion it is fairly wide, but narrows considerably previous to passing into the frontals. In the anterior portion, prior to its division into the two orbital branches, it widens again. The Supra-orbital Branch passes inwards and forwards after leaving the main canal, and as an almost straight branch passes to the tip of the snout, where it opens by a large pore posterior and dorsal to the anterior nares (PI. X X I . figs. 16, 17). The Sub-orbital Branch passes behind the orbit and some distance below it; before passing forwards beneath the orbit a large saccular dilatation passes backwards and opens by a large pore (PI. X X I . figs. 16, 17). Passing forwards to a region above and slightly in front of the angle of the mouth, another but shorter dilatation passes off, and, like the former, opens by a pore ; still further anterior is a third, into which the finger can be readily inserted. There is no pore opening from this. The branch now enters a cartilage and passes upwards and then downwards and forwards, and terminates by a large pore beneath the opening of the supraorbital branch. The Operculo-mandibular Branch commences immediately at the termination of the lateral canal. At its very commencement and just below the wall of the lateral canal is a pore which leads into a large saccular dilatation which passes backwards and slightly ventrally, terminating blindly. Passing downwards, enclosed in a series of cartilages, it again widens at its base into another much larger dilatation, which opens to the surface by a pore; slightly in front of this is another dilatation, somewhat smaller (PL X X I . fig. 17). The branch continues forwards, and, passing inwards, opens into the mandible, through which it passes, opening to the surface by four pores and meeting with its fellow of the opposite side. The connection is formed by a short canal in the cartilage. The mandible is peculiarly modified for the reception of the mandibular portion of the operculo-mandibular branch, a large portion of the ramus being broken up into a bony network, |