210 



THE WOKLD OF LIFE 



Fig. 44. — Homocercal (equal-lobed) Tail. 



Modern type of true fishes. 



The older types persist in some of the lower forms. (B.M. Guide.) 



This advance in development is well indicated by the 

 gradual changes in the tail, as shown in the accompanying 

 figures (42-44). The upper one is the oldest; but it soon 

 became modified into the second, which in various modifica- 

 tions prevailed throughout the Palaeozoic and most of the Sec- 

 ondary 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 ex- 

 ternally, 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 ex- 

 tremely 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 appearance 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 numer- 



