44 8 



HISTORICAL GEOLOGY 



Fig. 419. — Silurian 

 cephalopods : A, Daw- 

 sonoceras americanum; 



no important changes in the gastro- 

 pods (Figs. 418 B-E) are shown. 



Pelecypods (Fig. 418 A) also con- 

 tinued much as in the Ordovician. 



Cephalopods. — Curved and coiled 

 cephalopods (Fig. 419 B y C) were 

 more numerous than straight forms 

 (Fig. 419 A), while the latter were 

 smaller and were commonly orna- 

 mented by rings and low, trans- 

 verse ridges. The body chamber of 

 many Silurian cephalopods was con- 

 stricted (Fig. 419 B), the constric- 

 tion being apparently for the pur- 

 pose of protecting the soft parts of 

 the animal from enemies. As in 

 the Ordovician, this class was the 

 most powerful of the time. 



Arthropoda 



Trilobites. — This interesting class 

 (Fig. 420 A-D) was still important, 



B, Phragmoceras par- but the decline had already begun 

 vum; C, Trochoceras an( j i t was numerically decidedly 



desplainense. ■> • ^ • ^ r\ i 



less prominent than in the Ordo- 

 vician (Fig. 421). Since no new families appeared, the general aspect 



B \C D 



Iniinurus (Lichas) bigsbyi (boltoni) ; B, Ceratocephala 

 dujrenoyi; C, Dalmanites limulurus; D, Bumastus (Illcenus) ioxus. 



