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Show 1895.] AND AMPULLARY CANALS OP CHIMiERA. 881 canal across the throat, some distance behind the mandibular branch. In all the specimens I have examined this branch runs ventrally and backwards and then forwards, some distance from the region of the fin, in fact quite close to the mandibular branch. This condition was also common to the young examples. 3. T H E OCCIPITAL C O M M I S S U R E passes from the upward portion of the main canal of the head and has a slight backward flexure. Where the two sides meet there is sometimes a short median backwardly directed branch (PI. LI. fig. 4, Oc.com.). All previous authors are agreed as to the open-grooved nature of the canals in Chimcera, as distinct from the tubes found in Callorhynchus, and yet the canals do not always persist as open grooves in Chimcera or as closed tubes in Callorhynchus. In young examples of the former the canals are practically identical with those found in the adult; but in two adult specimens which I examined I noticed a portion of the lateral canal formed a perfect tube, previous to its connection with the main canal of the head, for a distance of about 27 millimetres. In the second specimen the upper portion of the sub-orbital branch was closed for about 12 millimetres. In neither case were there any signs of fusion, so that it is probable that the borders of the groove coalesced in a very early stage, or possibly they arose as distinct isolated tubes and became united with the open groove later, as the isolated portions of the canal do in more specialized fishes. In each case a fine wire was passed through the tube and then a larger one, in order to stretch it before it was finally cut through. In young specimens of Callorhynchus short portions of the lateral canal occasionally appear as grooves, the borders of which coalesce at a later stage and form distinct tubes. 3. Structure and Histology of the Canals. The minute structure of the canals and sensory organs is very similar to that described in other fishes-e. g. Leemargus and Raia, Ewart(6 & 7) *, Amia, Allis (1) ; Polyodon (3),-as a reference to the transverse section of the lateral canal will show (PI. LI. fig. 5). In certain portions of the canal-system, viz., the supra- and suborbital branches and the maxillary division of the maxillo-mandibular branch, there are a series of diamond-shaped openings, and where these are present the canals are deeper and considerably wider. Encircling these portions are a series of small cartilages having the form of a broken hoop, the two broken ends terminating as bulbous enlargements. Leydig (12) mentions that these cartilages often terminate in dendritic branches, but I have not observed this condition in any of the specimens I have examined, in fact little or no variation was noted in either their size or appearance. They measure 6 millim. in circumference and 1 millim. in breadth (PI. LIII. fig. 6). Sol eer (15) has carefully described the histology and certain modifications of the canals and sense-organs. |