THE ORDOVICIAN PERIOD 367 



by the incoming or outgoing of particular species or groups of species. These 

 represent the evolution of the fauna in a broader sense. For the greater part 

 these stages are yet to be worked out into precision and detail, though clearly 

 recognizable in general terms. 



4. The Upper Ordovician Fauna. — The closing fauna of the Ordovician in 

 North America retained most of the characteristics of the Middle fauna, from 

 which it was directly descended without radical modification due to interruption 

 of the controlling conditions. The chief change in the physical influences lay 

 in an increased prevalence of silt sedimentation and in the shallowing and final 

 withdrawal of the great epicontinental seas. With the development of the silt 

 sedimentation there came a very notable increase, relatively at least, in the 

 bryozoans (Fig. 161, a and b) and of the graptolites (Fig. 167, b, e, and /). The 

 pelecypods were favored by the mud bottoms and became prolific. More than 

 eighty species have been identified in a limited district in southern Ohio and 

 Indiana. 1 There was a relative decline in the number of gastropods, cephalopods, 

 and trilobites, and, in general, the fauna was perhaps less prolific; at least this 

 appears to be so from the larger proportion of matter of inorganic derivation 

 to that of organic derivation. 



The closing phase of the Upper Ordovician was the withdrawal of the fauna 

 of the interior as a necessary attendant of the withdrawal of the interior sea. 

 The succeeding fauna contained an almost entirely new assemblage of species, 

 though many of the genera remained the same. In the St. Lawrence embay- 

 ment, the Ordovician fauna lingered longer and was gradually changed into 

 or replaced by the Silurian fauna. Probably the same was true in other embay- 

 ments on the border of the continent and along its coast generally. 



The Ordovician Fauna of Other Continents. 



Much the same general lines of progress were followed on other continents, 

 and the general aspects of the successive subfaunas were much like those of 

 the American continent. Usually the genera were the same, but the species 

 as a rule were different, though they often bore a close resemblance to the Ameri- 

 can species. In northwestern Europe, with which the means of migratory com- 

 munication seem to have been the freest, not a few common American species 

 flourished, such as Dalmanella testudinaria, Platystrophia biforata, Dinorthis 

 porcata, Leptoena rhomboidalis, Plectambonites sericeus, Halycites catenulatus, and 

 Trinucleus concentricus. In Asia, so far as present limited information goes, 

 the species were nearly all different, the wide-ranging graptolites excepted, though 

 Halycites catenulatus and Plectambonites sericeus are found in China. The stages 

 of progress in the shallow-water faunas of the Old and New World are to be 

 regarded rather as parallel than as identical. The evolution in Europe, where 

 alone details have been well worked out, was usually on less broad lines than 

 that of the American interior, for the obvious reason that the epicontinental 

 seas were more limited and more interrupted by barriers. 



1 Ulrich, Geol. Surv. Ohio, Vol. VII, 1893, pp. 626-693. 



