THE ORDOVICIAN PERIOD 535 



of Cambrian faunas. These may be distinguished roughly as the 

 Lower, the Middle, and the Upper Ordovician faunas. In some 

 places, the late Cambrian faunas and the early Ordovician faunas 

 merge into one another without sharp definition. In general, the 

 Mid-Ordovician fauna was more prolific than that which preceded, 

 if we may judge from the fossils. The Mid-Ordovician fauna, too, 

 was distinctly cosmopolitan. The Upper Ordovician fauna was 

 similar to its predecessor, from which it descended, but the prevail- 

 ing muddiness of the bottom of the late Ordovician seas seems to 

 have had some influence on the life, and clear-water forms were less 

 dominant. 



The successive sub-faunas of the period were much the same in 

 other continents as in America. Most genera were the same, but 

 the species were, as a rule, different, though they often bore a close 

 resemblance to the American species. In northwestern Europe, 

 with which the means of migratory communication seems to have 

 been freest, not a few common American species flourished. In 

 Asia, so far as present limited information goes, the species were 

 nearly all different, the wide-ranging graptolites excepted. The 

 stages of progress in the shallow-water faunas of the Old and New 

 World, are to be regarded as parallel rather than identical. The 

 evolution in Europe, where alone details have been well worked 

 out, was usually on narrower lines than that of the American 

 interior, for the obvious reason that the epicontinental seas were 

 more limited and more interrupted by barriers. 



Map Work. The folios which serve for the study of the Cambrian system 

 ). 506) are serviceable also for the Ordovician. In the folios, however, the 

 rdovician and Silurian systems are put together on the maps, under the 

 ime Silurian; but the texts of the later folios distinguish between the two. 



