6 WATER REPTILES OF THE PAST AND PRESENT 



However, notwithstanding these imperfections of our geological 

 records, we know very much more about extinct reptiles than we 

 do about living ones, so far at least as those parts capable of pres- 

 ervation in the rocks are concerned. Were our knowledge of 

 reptiles confined to the forms now living upon the earth it would 

 be relatively very incomplete since, aside from the lizards and 

 snakes, they are merely the remnants of what was once a mighty 

 class of vertebrates. 



Not only do we learn from the remains preserved in the rocks 

 the precise shape and structure of the bones of the skeleton and 

 their precise articulations, but we are often able to determine not 

 a little regarding the forms which the living animals had by the 

 impressions made by the dead bodies in the soft sediment which 

 inclosed them before decomposition of the softer parts had ensued, 

 sediments which afterward solidified into hard rock. But these 

 impressions are, with rare exceptions, only those of profiles or of 

 flattened membranes. The rounded bodies of life do not retain 

 their shape long enough for the sediment to harden; in most 

 cases the flesh has decomposed before being entirely covered by 

 sediment. Sometimes the integument and scales in a carbonized 

 condition are actually preserved, retaining some of the actual 

 structure of the organized material. The carbon pigment of the 

 skin has sometimes been preserved in patterns indicating the color- 

 markings in some of these ancient reptiles; and even the micro- 

 scopic structure has been detected in carbonized remains of organs. 

 Fossil stomach contents, the bony remains of unhatched young, as 

 well as the delicate impressions of skin and membrane, all add to 

 our knowledge of the structure and habits of the animals which 

 lived so long ago. Many other things also may be learned, or at 

 least inferred, concerning the living animals and their habits from 

 the positions in which the skeletons are found, from the nature of 

 the rocks which inclose them, or from the character and abundance 

 of other fossils found with them. The frequent discovery of bones 

 which had been injured and mended during life, or the living ampu- 

 tation of members, often tell of the characteristics of the creatures. 

 So, too, the climatic conditions under which the animals lived may 

 often be inferred with tolerable certainty; the presence of " stomach 



