3 8o 



ORDOVICIAN PERIOD 



forms and coiled forms, some of which resemble the Nautilus of 

 to-day (Fig. 337, e). Straight forms predominated, however, 

 and the sutures (junctions of the septa with the outer shell) were 

 simple, in marked contrast with some of those of later periods. 



Fig- 337- ORDOVICIAN CEPHALOPODS: a, Poterioceras apcrtum Whiteaves; b, 

 Cyrtoceras neleiis Hall; c, Orthoceras bilineatum Hall; d, Oncoceras patidion Hall; e, 

 Trocholites ammonius Conrad;/, Orthoceras socialc Hall. 



In size, Ordovician cephalopods were probably never surpassed by 

 representatives of their 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. Unlike many 

 mollusks, cephalopods were free swimmers. Gastropods, the kin of 

 modern snails, were well represented in the early Ordovician fauna 

 by diverse forms (Fig. 338). Few types of early Paleozoic life so 

 closely resembled their modern relatives. Pelecypods (Fig. 338), 

 the class to which clams and oysters belong, were subordinate to 

 gastropods. Like their modern relatives, the Ordovician pelecy- 

 pods seem to have been fond of muddy and sandy bottoms, for 



