THE ORDOVICIAN PERIOD 



97 



of the few excellent illustrations of an important class of animals 

 which has shown surprisingly little change since early Paleozoic 

 time (Fig. 54). 



Cephalopods. "The largest, most powerful, and perhaps the 

 most predaceous of the known forms of Ordovician life were the 

 Cephalopods, which seem to have developed into prominence with 

 extraordinary suddenness. Unless the Fishes, of which very little 

 is known, contested their supremacy, they were doubtless the un- 

 disputed masters of the sea. Their relics first appear at the time 

 of the transition from the Cambrian to the Ordovician, but they 



Ordovician Cephalopods: a, Orthoceras sociale (Hall); b, Cyrtoceras neleus 

 (Hall); c, Trochoceras-like form (Silurian specimen after Barrande); d, 

 Trocholites ammonius (Hall). 



were then so far advanced and so widely differentiated from allied 

 forms as to render it probable that they had already lived a long 

 time. . . . The size attained by the Ordovician Cephalopods was 

 probably never surpassed by representatives of the class. Some of 

 the shells were 12 or 15 feet in length, and a foot (maximum) in 

 diameter. From this great size they ranged down to or below 

 the size of a pipe stem. " ! These Cephalopods all belonged to 

 the Tetrabranch or chambered-shelled subdivision of the class 

 (Fig. 55). 



1 Chamberlin and Salisbury: College Geology, pp. 525-527. 



