952 PRINCIPLES OF STRATIGRAPHY 



the southern United States and Central America, while other mem- 

 bers extend as far back as the Lias. 



The Dipnoi, or Lung-Fishes, range from the Devonic to the 

 present time. Their skeleton is chiefly cartilaginous, but the upper 

 and lower vertebral arches, the ribs and fin-supports exhibit a ten- 

 dency toward ossification. They have paddle-shaped, paired fins 

 and a highly specialized air-bladder which serves as a lung. Dental 

 plates are common in the Devonic (Dipterus) and Carbonic (Cte- 

 nodus), while many perfect specimens also occur in these deposits. 



These fish may be considered as approaching Amphibians in 

 many respects. The Teleosts, or bony fishes, appear first in the 

 Triassic deposits, and increase in prominence until they are the 

 leading type to-day. 



Amphibia. 



The Amphibia are cold-blooded terrestrial vertebrates, with 

 partly branchial respiration, in early stages, while in some forms 

 gills remain functional throughout life. The limbs are never fins 

 and are rarely absent. The StegocepJialia (Carbonic to LTpper 

 Trias) comprise the largest known Amphibians, and were pro- 

 tected by a dermal armor of bony scales or scutes. The teeth were 

 sharply conical, with a large pulp-cavity, and the walls were some- 

 times highly complicated by infolding of the dentine (Labyrintho- 

 donts). The Gymnophiona or ccecilians are vermiform amphibia, 

 covered with scales and without limbs. They are restricted to the 

 South American and Indo-African tropics. The Urodeles are 

 naked bodied, usually with two pairs of short limbs and persistent 

 tail. Gills often remain throughout life. The vertebrae are usually 

 completely ossified. This group appears first in the L^pper Jurassic 

 (Wealden), and has living representatives in the newts and sala- 

 manders. The Anura (frogs, toads) are tailless and develop by 

 metamorphosis. The oldest fossil forms are from the Eocenic. 



RepHlia. 



Reptilia are cold-blooded, naked, scaly or armored vertebrates 

 of terrestrial or aquatic habit, and breathing exclusively by lungs. 

 There is no metamorphosis during development. The Rhyncho- 

 cephalia date from the Permic, but were most extensively repre- 

 sented in the Trias. A single living genus (Hatteria or Spheno- 

 don) occurs in New Zealand. The body was lizard-like, long- 



