THE GEOLOGICAL RECORD 195 



tions prevailed throughout the Palaeozoic and most of the 

 Secondary periods ; while the third perfectly symmetrical 

 type did not appear till near the end of the latter, and only 

 became predominant, as it is now, in the Tertiary period. 

 Many of the earlier forms have tails which are quite 

 symmetrical externally, but show a slight extension of the 

 vertebrae towards the upper lobe. All three forms still 

 exist, but the third is by far the most abundant. 



In the highest Silurian beds land-plants allied to ferns 

 and lycopods first appear, and with them primitive 

 scorpions. In the succeeding Devonian and Carboniferous 

 strata an extremely luxuriant land vegetation of a low type 

 appeared and covered a large part of the existing lands. 

 This supported a large variety of arthropods as well as true 

 insects allied to Mayflies and cockroaches, with a great 

 number of Crustacea. Here, too, we come upon the next 

 great step towards the higher land animals, in the appear- 

 ance of strange Amphibia forming a distinct order — the 

 Labyrinthodontia. They appear to have outwardly resembled 

 crocodiles or lizards, and were rather abundant during the 

 Carboniferous and Permian eras, dying out in the subsequent 

 Triassic. 



That portion of the Palaeozoic series of strata from the 



Silurian to the Permian, during which a rich terrestrial 



vegetation of vascular cryptogams was developed, with 



numerous forms of arthropods, insects, primeval fishes 



ind amphibians, comprises a thickness of stratified rocks 



somewhat greater than that of the whole of the Secondary 



md Tertiary strata combined. This thickness, which can 



:>e measured with considerable approach to accuracy, is 



generally supposed to afford a fair proportionate indication 



)f the lapse of time. 



There is a popular impression that in these remote ages 

 he forces of nature were more violent, and their results 

 nore massive and more rapidly produced, than at the 

 •resent time ; but this is not the opinion of the best 

 eological observers. The nature of the rocks, though often 

 hanged by pressure and heat, is in other cases not at all 

 ifferent from those of subsequent ages. Many of the deposits 

 ; ave all the characters of having been laid down in shallow 



