346 



There is evidence that the great vertebrate branch had 

 become distinct as early as the Ordovician period, for scales, 

 which appear to be those of fishes, have been 

 found in rocks of that age in the Rocky 

 Mountains. Still another long period must 

 be passed, however, before fishes come into 

 prominence. 



Land plants and animals. Considering the 

 fact that the continents were so largely sub- 

 merged and that the known Ordovician strata 

 in which our only record of 

 the life is preserved are of 

 FIG. 353. One marme origin, it is not sur- 



of the earliest . . . 



and simplest prising that the land animals 

 cephalopoda anc j pl an ts of this period are 



(Orthoceras). , , , , 



scarcely better known than 



are those of the Cambrian. An insect's 



wing from the rocks of Sweden proves that 



the land-inhabiting arthropods had already 



come into being, and it adds confirmation to 



our previous 

 suspicion that 

 land vegeta- 

 tion existed in 

 those early 

 times ; for the 



winged insects are almost 

 wholly dependent upon plants 

 for their sustenance. 



Crustal disturbances at the 

 close of the period. The long, 

 quiet reign of the epiconti- 

 nental seas, which had begun 

 in the Cambrian and continued 



FIG 355. -A coiled Ordovician through the Ordovician, was 

 cephalopod related to forms still 



FIG. 354. Broken 

 cast of a curved 

 Ordovician ceph- 

 alopod (Cyrto- 

 ceras). Compare 

 the sutures with 

 those in Figures 

 391 and 418. 



living. 



partially interrupted by events 



