THE ORDOVICIAN PERIOD 



527 



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

 simple. In later periods the sutures vary widely, and marked, in 

 a very tangible way, the progress of the class. 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. 



Other Molluscs. The gastropods were well represented in the 

 early Ordovician fauna by diverse forms (Fig. 388). Few types 



Fig. 389. Ordovician Pelecypods: a, Pterinea demissa (Conrad); 6, Bys- 

 sonychia radiata (Hall); c, Vanuxemia dixonensis M. and W., interior 

 of right valve, showing the hinge and muscular impressions; d and e, 

 Ctenodonta nasuta (Hall); /, Lyrodesma cincinnatiensis Hall, interior 

 of right valve, showing a primitive type of hinge; g, Ctenodonta recurva 

 Ulrich; h, Ctenodonta pectunculoides Hall; i, Rhytimya radiata Ulrich, 

 exterior of right valve. 



of early Paleozoic life so closely resembled their modern relations. 

 The pelecypods were subordinate to the gastropods both in numbers 

 and range. Representative forms are shown in Fig. 389. Like 

 their modern relations, the Ordovician pelecypods seem to have 

 been fond of muddy and sandy bottoms, for they are rather rare 

 in the limestone beds of the early and middle Ordovician. They 

 increase in abundance as the deposits grade into the shales of the 

 later Ordovician. 



