AQUATIC REPTILES 



203 



Descendants of these secondarily armored, shore-living types 

 again sought the sea and entered a secondary marine pelagic 

 phase in course of which they lost the greater part of their 



REPTILIA ARCHELON cRE^tTcIou: REPTILIA PLACOCHELYS 



Fig. 80. Armored Terrestrial Cheloxia ^^ 

 In\'.«)e the Seas and Lose Their Araia- f 



TURE. 



Convergent or analogous evolution (two 



upper figures) in the inland seas of the 



paddle-propelled chelonian Archelon (after 



Williston), the gigantic marine turtle of 



the Upper Cretaceous continental seas of 



North America, and of Placochclys (after 



Jaekel in part), a Triassic reptile belonging 



to the entirely distinct order Placodontia. 

 Skeleton of Archelon (lower) in which the 



bony armature of the carapace has largely 



disappeared, exposing the ribs. Specimen 



in the Peabody Museum of Yale Univer- 

 sity. After Wieland. 



second armature and acquired their present leathery covering, 

 to which the popular name ''leatherbacks" applies.^ 



In general the law of reversed aquatic adaptation is most 

 brilliantly illustrated in the fossil ichthyosaurs, in the internal 



' This law of alternate adaptation may be regarded as absolutely established in the 

 case of certain land-living marsupials in which anatomical records remain of an alterna- 

 tion of adaptations from the terrestrial to the arboreal phase, from an arboreal into a 

 secondary terrestrial phase, and from this terrestrial repetition to a secondary arboreal 

 phase. The relics of successive adaptations to alternations of habitat zones and adap- 

 tive phases are clearly observed in the so-called tree kangaroos {Dendrolagiis) of Australia. 



