COLOSSOCHELYS ATLAS. 371 



from the design, appears to have been that the tortoises 

 derived their fluorine from the vicious habit of smoking 

 tobacco ; but I beg to repudiate any comiection with such an 

 opinion. 



The following is Mr. Middleton's Analysis of the Plastron of 

 the Colossochelys Atlas^ : — ' Yiewed through a magnifying lens, 

 minute crystals, and here and there nests of minute crystals 

 of carbonate of lime are seen to occupy the pores of the fossil 

 associated with oxide of iron, which latter gives to it a 

 slightly brownish tinge. It is readily soluble in nitric and 

 hydrochloric acids. Five grains, finely powdered, were slowly 

 raised to a red heat, and continued in that state for some 

 time without having undergone from first to last any change 

 of colour. It was found, on re-weighing, to have lost ^-L- of a 

 grain, a loss so small that it might easily have arisen from 

 other causes than diminution in the weight of the substance. 

 A piece, if exposed to the fumes evolved in the usual way, gave 

 with remarkable readiness and decision indications of the 

 presence of fluorine. 



Judging from the quantity of fluorine j^resent, I am disposed 

 to assign to the existence of this animal a date much anterior 

 to that of the Mastodon Elephanto'ides, as the bones of the 

 former contain nearly twice as much fluorine as those 

 of the latter. How far dissimilarity of structure in the 

 tooth of the Mastodon (which I analyzed) and the plastron of 

 the Tortoise may go to account for the disparity in the quantity 

 of fluorme in them ; how far, for instance, the latter be better 

 suited for the absorption or development of fluorine than the 

 former, I am not prepared to say ; but as no reason suggests 

 itself to me, other than that of the lapse of time, which abundant 

 experiments have shown to be attended with the presence of 

 that substance in bones, I do not think it necessary to with- 

 hold the conclusion at which I have arrived. 



The analysis was performed by me in the laboratory of 

 University College, with every possible care, and for greater 

 certainty repeated. The following are the results : — 



Per cent. 

 Phosphate of lime ....... 64-95 



Carbonate of lime ....... 22-36 



Fluoride of c.ilciiun ....... 11-68 



Oxide of iron 1.00 



A trace of chloride of sodium. 



99-99 



' Found among Dr. Falconer's papers. — [Ed.] 



B B 2 



