352 AN INTRODUCTION TO THE STUDY OF FOSSILS 



Order i, Stegocephalia 



Extinct, tailed Amphibia, often very large. There are, 

 as a rule, two pairs of Hmbs present and usually a ventral 

 external armor of plates over the thorax, and behind these 

 overlapping scales; more rarely scales are present upon the 

 back. 



These are included with the Amphibia because (i) gill-arches 

 are present in immature skeletons ; (2) they breathed by lungs 

 alone in adult, or by lungs and gills ; (3) the ribs do not encircle 

 the thorax ; (4) the mucus-canal system was well developed as 

 indicated by deep impressions, especially observed in the bones 

 of the head. They show probable derivation from the crossop- 

 terygian fishes in that (i) the conical teeth are often complexly 

 infolded {e.g. the labyrinthodonts) as in some Paleozoic fish {e.g. 

 Holoptychius) ; (2) a ring of bony plates is present around the 

 eye-sockets ; (3) a pineal foramen is present ; (4) the bones of 

 the skull roof are similar in arrangement. 



They lived in fresh water, though some may have been more 

 or less completely terrestrial. In the Joggins coal mines 

 (Pennsylvanian) of Nova Scotia such genera as Dendrerpeton 

 and Hylonomus are common in decayed trunks of Sigillaria 

 and Lepidodendron associated with the fresh-water gastropod 

 Dendropupa vetusta. They were abundant in the Texas- 

 New Mexico region during the Upper Pennsylvanian and Lower 

 Permian times (Fig. 150). 



Stegocephalia have been recognized from the Mississippian 

 to the Triassic. Branchiosaurus is a well-known genus from 

 the Permian and the labyrinthodont genus Mastodonsaurus 

 from the Triassic. The latter is the largest amphibian known, 

 with a skull over four feet long and almost as wide. 



Order 2, Urodela {Salamanders) 



Tail present throughout life (whence the name from Greek 

 oura, SL tail, + delos, apparent). Two pairs of approximately 



