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 
