130 ANIMALS OF THE PAST 



Spines and plates usually leave some trace 

 of their existence, for they consist of a super- 

 structure of skin or horn, built on a founda- 

 tion of bone ; and while even horn decom- 

 poses too quickly to " petrify," the bone will 

 become fossilized and changed into enduring 

 stone. But while this affords a pretty sure 

 guide to the general shape of the investing 

 horn, it does not give all the details, and there 

 may have been ridges and furrows and sculpt- 

 uring that we know not of. 



Knowing, then, what the probabilities are, we 

 have some guide to the character of the cover- 

 ing that should be placed on an animal, and if 

 we may not be sure as to what should be done, 

 we may be pretty certain what should not. 



For example, to depict a Dinosaur with 

 smooth, rubbery hide walking about on dry 

 land would be to violate the probabilities, for 

 only such exclusively aquatic creatures as the 

 whales among mammals, and the salamanders 

 among batrachians, are clothed in smooth, 

 shiny skin. There might, however, be reason 

 to suspect that a creature largely aquatic in its 

 habits did occasionally venture on land, as, for 



