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Show 1895.] SENSOEY CANAL SYSTEM OE FISHES. 277 The Sub-orbital Branch passes clown the anterior border of the sphenotic and through the post-orbital. At the commencement of its course in the post-orbital it gives off the backwardly directed branch terminating at pore 9. Continuing forwards it gives off another similar branch in the sub-orbital bone, which terminates at pore 10. It passes then into the lachrymal, opening by a pore on the suture, and then into the maxilla, where it bifurcates and terminates by the two pores 13 and 14 (fig. 1). The Operculo-mandibular Branch leaves the main canal of the head in the anterior portion of the squamosal, from which it passes into and through the pre- and inter-operculum; in the former it branches, and opens in the latter by pore 2. Leaving the inter-operculum it becomes connected with the mandible by a fine dermal canal. It traverses the whole length of the mandible and in the most anterior portion meets with its fellow of the opposite side. In its course from the main canal of the head to the symphysis of the mandible it opens by eight pores. In Choetostomus, according to Pollard (11. p. 538 and p. 543), the mandibular branch is absent. 3. T H E C O M M I S S U B E . - T h e only commissure present in Clarias nieuhofii is that in the frontals, which establishes a connection between the canals of either side of the head. Pollard (11. p. 527) states that this commissure " is distinctly rudimentary and gives very strongly the impression that it formerly united with its fellow of the opposite side." Again, on p. 542 op. cit. he states " it is a complete commissure, but one becoming somewhat rudimentary." He does not state what species he examined, but in neither of those I have examined are there any signs of it becoming rudimentary. I note further that he omits in his figure (11. pi. 35. fig. 1) any sensory organs in this commissure, one being present in Clarias magur on either side of the median lines and innervated by the ramus ophthalmicus superficialis of the trigeminal nerve. INNEEVATION. Clarias magur. The nerves innervating the sensory canal system are the trigeminal, facial, glossopharyngeal, and vagus. Of the trigeminal group the following branches innervate the canals and sensory organs:- 1. The ramus ophthalmicus superficialis. 2. The ramus buccalis. 3. The ramus oticus. 1. The ramus ophthalmicus superficialis is the most dorsal branch of the trigeminal and passes directly forwards. It gives off branches to all the sense-organs lying between the pores 16 to 19 on the supra-orbital branch. Slightly posterior to the frontal commissure a short branch passes off which innervates a sense-organ on the lateral border of the commissure. In the anterior |