220 Dr Hibbert on the Limestone of Burdiehouse, 



There is a small proportion of soda in the recent bones, and perhaps a less pro- 

 portion of potash and soda existing partly as chlorides in the Gyracanthus. 



The proportion of animal matter in the bones of the pike amounts to as much as 

 37.86 parts in a hundred, of which a trace only appears in the Gyracanthus for- 

 mosus. 



Having stated these differences, I shall first make some remarks on the great 

 quantity of phosphate of lime possessed by the fossil ray. 



Mr Connell has very properly remarked to me, in reference to the Gyracanthus 

 having been assigned to the family of the Cestraciontes, that the Cestracion is consi- 

 dered as a cartilaginous fish, whilst the great quantity of bone-earth contained in 

 the dorsal ray of the fossil animal, amounting to 54 per cent., is incompatible with 

 the notion of its being any thing but bone. Now, I certainly find it remarked of 

 cartilaginous fish, that, while the skeleton of these animals remains constantly carti- 

 laginous, in rare cases only it contains osseous fibres ; the calcareous matter being 

 in such instances deposited simply in the form of small grains. There are, at the 

 same time, certain fish of this class, though appearing to separate themselves from 

 it, as, for instance, the orders of Chismopnea and Teleobranchia, among which the 

 skeleton is fibrous, and becomes with age perfectly osseous. 



To such a structure, therefore, I would refer the fossil dorsal rays of the Gyra- 

 canthus, which appear to have been formed of osseous fibres, disposed in a longitudi- 

 nal manner (as can even be traced in their mineralized structure), the intervals or 

 pores having been originally filled with animal matter. This arrangement of struc- 

 ture is indeed remarkably shewn in some fossil specimens, where elongated pores, or 

 tubes, larger than common, are filled with siliceous matter. 



Another remark is suggested by the mode in which the loss of animal matter has 

 been supplied in the fossil rays of the Gyracanthus. It would here seem, that the 

 bones of the pike contain, in addition to the phosphate of lime, about six parts in a 

 hundred of the carbonate of lime. Now, it is remarkable that the fossil thorny ray 

 contains as much as 34 parts nearly of the carbonate of lime, along with only 10 

 parts of siliceous matter, by which it appears that the loss of animal matter has been 

 very readily supplied by infiltrations of calcareous matter, derived no doubt from a 

 calcareous matrix, but comparatively little with siliceous matter. Ought this circum- 

 stance to throw any light upon the original organization of these rays ? 



There is, at least with myself, much difficulty in explaining why the loss of animal 

 matter is chiefly supplied in these fossil rays by carbonate of lime, which is exactly 

 the reverse of what appears to have taken place in fossil scales. Whether, in this 

 instance, the animal matter has been sooner released from the substance of the bone, 

 and, as a consequence, its place more directly supplied by infiltrations of calcareous 

 matter from the matrix of these relics, I will not pretend to determine. In the dense 

 and compact structure of the bony scales of the Megalichthys, the mineralization 

 might have gone on far more slowly ; and hence the substitution of a great excess of 

 siliceous matter. 



