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BIRD-LIKE REPTILES. 225 
still living alligators are first met with in a fossil state 
in the chalk and tertiary strata. The most isolated of 
the four existing orders of reptiles consists of the re- 
markable group of Tortoises (Chelonia); fossils of these 
strange animals are first met with inthe Jura. In some 
characteristics they are allied to Amphibia, in others, to 
Crocodiles, and by certain peculiarities even to Birds, so 
that their true position in the pedigree of Reptiles is 
probably far down at the root. The extraordinary re- 
semblance of their embryos to Birds, manifested even at 
later stages of the ontogenesis, is exceedingly striking. 
The four extinct orders of Reptiles show among one 
another, and, with the four existing orders just mentioned, 
such various and complicated relationships, that in the 
present state of our knowledge we are obliged to give up | 
the attempt at establishing their pedigree. The most 
deviating and most curious forms are the Flying Reptiles 
(Pterosauria) ; flying lizards, in which the extremely elon- 
gated fifth finger of the hand served to support an enormous 
flying membrane. They probably flew about, in the 
secondary period, much in the same way as the bats of the 
present day. The smallest flying lizards were about the 
size of a sparrow; the largest, however, with a breadth of 
wing of more than sixteen feet, exceeded the largest of our 
living flying birds in stretch of wing (condor and albatross). 
Numerous fossil remains of them, of the long-tailed Rham- 
phorhynchia and of the short-tailed Pterodactyle are found 
in all the strata of the Jura and Chalk periods, but in these 
only. 
Not less remarkable and characteristic of the Mesolithic 
epoch was the group of Dragons (Dinosauria, or Pachypoda). 
