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. (1| in.) in their greatest 

 length ; their average length is about 28*8 mm. (1| 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 whei-e 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 transverse 

 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 

 raucous 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./.), 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 %. 



Mineral matter 67-189%. 



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. 



