THE PENNSYLVANIAN PERIOD. 607 



result. The emergencies of shoaling water and shifting drainage 

 must inevitably have called into play such powers of air-breathing 

 as the fishes of the time possessed, as well as such ability as 

 they may have had to use the paired fins in locomotion on the 

 land. The great development of the lung-fishes in the Devonian is 

 perhaps one result of this, and they have been regarded as the pos- 

 sible ancestors of the amphibians. This view is now largely aban- 

 doned for anatomical reasons, and some branch or branches of the 

 crossopterygians regarded as the more probable ancestors. The con- 

 necting link, however, yet awaits discovery; but evolution from some 

 form of fish as early as the Devonian period, if not earlier, is re- 

 garded as extremely probable. Because of the fatal effects of salt 

 water on modern amphibians, and for other reasons discussed in 

 connection with the Devonian fish, there is a presumption that the 

 evolution took place in and from fresh water, not unlikely in the 

 continental interior where fluctuations in drainage and precipita- 

 tion might well have been greater than on the coast, and hence the 

 scantiness of the transition record. Even in the early Carboniferous 

 the record was poor, and relics of amphibians appear in abundance 

 only in the later Coal Measures. They were then already differentiated 

 into five suborders, Branchiosaaria, Aistopoda, Microsauria, Temno- 

 spondyli, and Labyrinthodonta. 1 These were all still rather primitive 

 in structure, but they were quite far from being the real transition 

 forms. They are the most primitive terrestrial vertebrates known at 

 present, and if not really the ancestors of all later land vertebrates, 

 they probably represent their general nature. They all seem to have 

 been low, elongate forms of salamandrine aspect, and are classed as 

 stegocephalians (roofed-head), their heads being well roofed over by 

 the bony plates of the skull. They have also been termed labyrintho- 

 donts, from the intricate infolding of the dentine of the teeth of one 

 of the suborders, but the others do not possess the character. 



The Branchiosaaria were perhaps the most primitive in structure, 

 though they have not been found earlier than some of the more ad- 

 vanced types. They were small and salamander-like in form, with 

 short, stumpy, weak limbs and abbreviated tails. The teeth were 

 simple, hollow cones, not labyrinthine. Forms in all stages of growth 



1 The classification here followed is slightly modified from that of Smith Wood- 

 ward, Vert. Pal., from whom many of the following facts are drawn. 



