MESOZOIC AND CENOZOIC LIFE 431 
them have teeth and other reptilian characteristics, 
(Figs. 254-256) so that one is very much in doubt 
whether they (such as the Archzopteryx (Fig. 255)) 
are really birds with teeth, or reptiles with feathers. 
They furnish one of the best illustrations of evolution 
so far found in the record 
of life preserved in the rock. 
Mammals of low types 
first appeared in the Ju- 
ratrias time, and by the 
close of the Mesozoic, this 
group of animals is well 
represented upon the earth. 
In this country the earliest 
mammals have been found Fic. 249. 
in the Triassic beds of Section in the middle of Fig. 248, show- 
North Carolina. They be- ak ee . aes a cee 
jong came lower orders Se" Sow Partly we 
(Marsupials and Mono- 
tremes) of mammals, which are now represented on the 
earth only by a few species, of which the kangaroo 
and the duck-billed platypus of Australia are examples. 
While it is not certain what the ancestry of these 
ancient mammals is, there is some reason for think- 
ing that they have descended from the Amphibia. 
The lowest forms of mammals lay eggs very much 
in the same way that these low vertebrates do. 
