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Show 26 MR. A. D. IMMS ON THE GILL-RAKERS [May 3, appears to be to prevent the gill-rakers from becoming damaged in the way suggested, and to enable them to be stowed away in a regular order, and within a small compass, so as to admit of the closing of the operculum in the acts of respiration. In an example of Polyodon the length of which measured 166 cm. (5 ft. 4 in.) from the tip of the rostrum to the extremity of the tail, the longest gill-rakers measured 45 mm. ( I f in.) in their greatest length ; their average length is about 28'8 mm. (lg in.). It will be seen upon referring to fig. 3 (PI. II.), that each gill-raker consists of two parts, viz. a slender shaft, which tapers gradually towards, its free extremity, and a basal portion, which is embedded under the mucous membrane covering the branchial arch. The basal portion in the specimen figured measures 8 mm. long and 1‘5 mm. in its greatest breadth; the shaft near to where it joins the base measures '75 mm. across. Each gill-raker is flattened from side to side at the base, while the shaft is nearly square in ti'ansverse section. In their natural position, the gill-rakers are disposed with their flat surfaces at right angles to the septum and are packed very closely together, the interval between any two scarcely measuring -25 mm. In colour they are pale yellow-brown, and many of them are somewhat darker at their extreme points. Their surface is smooth and shining; they are extremely brittle, and when viewed with transmitted light they have a translucent appearance. When dried they are bone-coloured and perfectly opaque. The method of attachment of the gill-rakers to the branchial arch is shown in PI. II, fig. 5, where the lower part of one is represented. It will be seen that the basal portion is inserted just under the mucous membrane (m.) covering the branchial arch, and that it lies parallel with the cartilaginous septum. It is attached to a branchial arch by numerous elastic fibres (e.f.), which are firmly inserted into the gill-raker and form a remarkably tough, strong ligament. Some additional fibres (e.f.') closely ensheath the base at its lower extremity, and others serve to unite adjacent gill-rakers to one another. Inserted on the outside of each are some muscle-fibres (m.f.) which pass downwards and inwards to be attached to the cartilage of the branchial arch. A sample consisting of '869 grm. of the shaft portions of gill-rakers, after having been washed in distilled water and then thoroughly dried, yielded upon analysis the following chemical composition * :- Organic matter ....... 32*811 °/0. Mineral matter ....... 67,189°/0- The mineral matter consists of calcium phosphate 26‘80 %, together with carbonates, fluorides, chlorides, and sulphates of calcium, with a little magnesium and iron. On account of the small amount of calcium phosphate they * I am indebted to Mr. C. J. Thompson, of the Chemical Laboratory of the Birmingham University, for undertaking this analysis. |