108 VERTEBKATA. 



Mesozoic rocks, however, tlie development of this class was 

 so great that the Secondary period has been termed the "Age 

 of Eeptiles." The class Avcs is doubtfully represented by 

 footprints in strata of the age of the Trias ; but no Palffi- 

 ozoic remains of this class have been as yet detected. The 

 earliest undoubted remains of Birds occur in the Jurassic 

 series, and the class has continued to be represented more or 

 less abundantly to the present day. Lastly, the class of the 

 Mammalia, so far as at present known, finds its earliest fossil 

 representative in strata of the age of the Trias {New Eed 

 Sandstone). The Mammals, however, cannot be said to be 

 in any way abundant as fossils, till we reach the Eocene 

 Tertiary. From this point onward the remains of Mammals 

 are as abundant as, in the nature of the case, they could 

 reasonably be expected to be. 



