1904.] OF THE SPOONBILL STURGEON. 2? 



contain, the gill-rakers differ very greatly in their chemical 

 composition either from bone or dentine. 



I found that the gill-rakers, after being decalcified and then boiled 

 for a few minutes in a strong solution of potassium hydroxide, 

 retained their general form, but the only sign of structure visible 

 in them was a coarse fibrous groundwoi-k. After a prolonged 

 treatment with potash this disappeared, and all that remained 

 was a small amount of a gelatinous precipitate. 



If the shaft of a gill- raker be thinned by rubbing down on the 

 surface of a fine hone, and then mounted in Canada balsam, a 

 good deal of its minute structure can be made out. When 

 viewed under a magnifying- power of 40 diameters, it is seen to 

 consist of a transparent, faintly yellowish ground-substance, 

 scattered through which are great numbers of lacunae with 

 canaliciili radiating from them. Running through the matrix or 

 ground-substance in a longitudinal direction are yellowish-brown 

 canals which contain blood-capillaries (PI. II. fig. 6). In thin sec- 

 tions the ground-substance appears quite colourless, and when seen 

 under a fairly high magnification indications of lamination are 

 visible in it at the periphery, but they die out towards the centre. 

 The lacunse are for the most part slightly larger than those which 

 are seen in a section of a human long-bone, and they bear no 

 definite relation to the blood-channels, so that there are no 

 indications of Haversian systems. The canaliculi are not 

 nearly so numerous as those found in typical bone, but they are 

 more frequently branched, and, moreover, they principally arise 

 from the two opposite ends of a lacuna, and those belonging to one 

 lacuna freely anastomose with those of several of the neighbouring 

 ones (fig. 7). A very marked tendency is exhibited by the lacunae 

 to be disposed with their long axes parallel to the surface of the 

 shaft with their canaliculi running in a similar direction. The 

 channels containing the blood-capillaries, which are most numerous 

 towards the base of the shaft (fig. 6), take the form of longitudinal 

 canals anastomosing with one another by means of short lateral 

 connections which are given off at frequent intervals. Traced 

 higher up the shaft, the number of these canals becomes consider- 

 ably less, and the anastomoses with neighbouring canals become 

 fewer. As the extremity is reached they become reduced to two 

 or three trunks, which eventually join with one another just under 

 the extreme point (fig. 8). The exact method of the termination 

 of the channels, however, is not easily to be seen in spirit material, 

 owing to the small amount of blood that is present in the capillary 

 vessels which are contained within them. In the basal portion of 

 the gill-raker vascular channels are entirely absent, but running 

 down the centre is a long, narrow, apparently empty cavity. The 

 matrix is homogeneous, and exhibits in places faint traces of 

 lamination. It is well supplied with lacunae, and many of them 

 differ from those found in the matrix of the shaft in having a 

 knotted or less regular outline, and in being more profusely 

 branched. 



