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880 MR. WALTER E. COLLINGE ON THE SENStiRY [Dec. 17, branch divides into maxillary and mandibular divisions; anteriorly the former division meets with the sub-orbital branch, previous to which, however, it gives off a ventral flexure. N o short or dendritic branches are given off from either the lateral or main canals or the larger branches. There is also an absence of primitive pores, and the cluster-pores are very few in number. In certain parts of the canals of the head there are large diamond-shaped openings. In these regions the canal is distinctly larger and is supported and protected by a series of cartilages. 2. Course of the Canals and Branches. 1. THE LATERAL CANAL commences a few millimetres from the end of the long whip-like prolongation of the tail. Its course lies on the lower side of the muscles of the trunk. Rising slightly dorsally, it passes on to the sides of the body and continues forwards as an open groove to the region of the head. There are no branches of any description passing off from the lateral canal, a feature so characteristic of many Elasmobranchs. 2. T H E M A I N C A N A L O E T H E H E A D passes directly dorsally, giving off an occipital commissure, and then passing forwards and slightly inwards. Where the main and lateral canals meet with the sub-orbital branch, in some of the specimens examined a small forwardly directed branch was given off (PI. LI. fig. 2, x.). The Supra-orbital Branch commences where the main canal of the head turns as a forward and slightly outward branch. It passes above the orbit to the tip of the snout, and then makes a sharp turn and meets with the sub-orbital branch (PI. LI. fig. 1, Sp.o.). The Sub-orbital Branch leaves the main canal of the head at its commencement and passes ventrally, giving off a short jugular branch, and immediately in front of this the maxillo-mandibular branch. The sub-orbital branch continues forwards making a somewhat S-shaped curve and meets with the supra-orbital branch. At the point where these two branches become connected there is a Y-shaped commissure connecting the supra- and sub-orbital branches of either side of the head with the maxillary division of the maxillo-mandibular branch (PI. LI. fig. 1, S.or.). The Maxillo-mandibular Branch is given off from the sub-orbital branch. The maxillary division passes above the upper jaw, meeting with its fellow of the opposite side. It also gives off dorsal to this, on either side, a short branch which meets with the base of the Y-shaped commissure previously described (PI. LI. fig. 1, Mx. & Mn.). The mandibular and jugular branches are evidently subject to much variation. In most of the specimens I have examined the former do not meet at the symphysis, but terminate at either side of the mouth ; in others there is an interrupted canal, as figured by Garman (9. pi. ii. fig. 5). The jugular branch this author figures as passing backwards to the region of the pectoral fin, where it makes a sharp inward curve and runs as an interrupted |
