I036 PRINCIPLES OF STRATIGRAPHY 



reaching the Amblystoma stage. The Urodeles began in the Cre- 

 tacic and continue to the present. 



The Aniira (frogs and toads) pass through a tadpole stage, 

 during which they breathe by means of gills and lead an aquatic 

 life. On transformation they lose the gills and the tail and become 

 air-breathers, though frogs can remain under water for a very 

 long period of time, absorbing oxygen through the skin. The 

 Anura are wholly post-Mesozoic in age. No marine amphibians 

 are known. 



Reptilia. The Rhynchocephalia comprised terrestrial and aqua- 

 tic forms, often of great size. The Squamata include two Cretacic 

 groups of marine reptiles, some of which, like the mosasaurs, were 

 of large size. The lizards (Lacertilia) and snakes (Opiiidia) are 

 chiefly terrestrial reptiles, though some of the latter (Hydr op hides) 

 live in the water. Some Pleistocenic species attained a length of 

 lo meters, but modern forms are all small. The lizards are 

 mostly provided with legs, while the snakes are legless. At the 

 present time, lizards are chiefly restricted to the warmer regions 

 of the earth's surface. Comparatively few forms pass beyond the 

 fortieth parallel, while above the sixtieth parallel lizards are prac- 

 tically absent ; though Laccrta vivipara, a species ranging over 

 nearly the whole of Europe, extends northward to the seventieth 

 parallel in Norway, and, together with the blind-worm (Angius 

 fragilis ) occurs in Lapland but is absent from the New World. Both 

 groups prol)ably appeared in the Cretacic. The fish-lizards (Ich- 

 tliyosauria and Sauro pterygia) were wholly nektonic in habit, liv- 

 ing in the sea, but breathing by means of lungs. They were, there- 

 fore, neither true holo-nekton nor atmo-nekton, but a transitional 

 type between the two, as are other vertebrates which lead a per- 

 manently nektonic life (e. g., whales, etc.). Some species inhabited 

 fresh or brackish water. 



The Theromorphs were habitually walking animals. It is be- 

 lieved by many that the mammals have arisen from these reptiles. 



The Chelonia or turtles are terrestrial or aquatic in habit, but, 

 like all other reptiles, are air-breathers. A few types are exclusively 

 marine, but the larger number live in fresh-water lakes or rivers. 

 It has been supposed that the ancestral species were aquatic, living 

 in swamps and shallow water, like modern crocodiles ; and that 

 from these descended the fluviatile types, from which in turn were 

 derived the early marine types, on the one hand, and the terrestrial, 

 on the other. 



The modern crocodiles are aquatic reptiles living in fresh water, 

 swamps and streams. The oldest Triassic crocodiles (Parasuchia 



