BOOM in. FOSSIL BOXES OF REPTILES. 149 



FOSSIL BOXES OF REPTILES. The state of mineralisation 

 of the fossils we have now to examine, differs considerably 

 from that presented by the ornithic relics from New Zealand. 

 The osseous carapaces and plastrons of the turtles, and the 

 bones and teeth of the crocodiles and lizards, are almost 

 without exception heavy, and of various shades of brown or 

 umber, from the permeation of their structures by solu- 

 tions of carbonates or oxides of iron. 



In some instances, bones of a jet black are imbedded in 

 white calciferous grit; the phosphoric acid in the original 

 organism having combined with iron and produced a deep 

 blue or black phosphate of that mineral, and left the sur- 

 rounding stone uncoloured. 



Infiltration of calcareous spar is a mode of fossilisation 

 equally common ; and the cancellous structure, and the 

 medullary cavities of the long bones, are very often filled, 

 more or* less, with white calc-spar; brilliant pyrites also 

 enters into the composition of these fossils, frosting over with 

 a golden metallic deposit the linings of the cavities and fissures. 



The petrifaction of the teeth by mineral matter, produces 

 beautiful examples of the intimate tissues of those organs ; 

 the dentine is often stained throughout with a rich sienna 

 tint, and sections viewed under the microscope by transmitted 

 light, reveal the character and distribution of the calcigerous 

 tubes more denned even than in recent specimens. 



It is extremely rare that osseous structures are found sili- 

 cified, or, in other words, petrified by flint ; among the many 

 thousands of bones which I have extricated from the rock, 

 or have seen in collections, I know but of a solitary instance, 

 a caudal vertebra of a Mosasaurus, which I obtained from 

 a chalk-pit near Brighton. But notwithstanding the weight 

 and apparent solidity imparted by these modes of minerali- 

 sation, the osseous substance is generally rendered extremely 

 brittle, so that the development of the bones from the stone 

 in which they are imbedded, and the removal of the hard fer- 

 rugino-calcareous crust investing them, is no easy task, but 

 requires much tact and experience and patience to execute 

 successfully. 



The observer, therefore, must not suppose that specimens 

 like the fossil Turtles and Reptiles in the first Cases, or 

 the colossal bones from the Wealden, in Case B, or the 



