THE CAMBRIAN PERIOD 



75 



Mollusks. — All the principal fossil-forming types of Mollusks 

 were represented. Pelecypod shells are small and comparatively 

 rare, being mostly found in the Lower Cambrian (Fig. 37) . From 

 Cambrian time to the present they have steadily increased, both as 

 regards number of species and individuals. In later geologic times 

 the shells often attained great size. Gastropods, mostly of simple, 

 low-conical types, were fairly common throughout the period 

 (Fig. 38). Cephalopods comprise the highest class of Mollusks 



Fig. 39 

 Cambrian Trilobites, restored forms : a, Olenellus gilberti, characteristic of the 

 Lower Cambrian; b, Paradoxides bohemicus, characteristic of the Middle 

 Cambrian; c, Dikellocephalus pepinensis, characteristic of the Upper Cam- 

 brian. (From Chamberlin and Salisbury's "Geology," permission of Henry 

 Holt and Company.) 



and they have been rarely found only in Middle Upper Cambrian 

 strata. They were simple, straight, or curved, chambered-shelled 

 forms. The Cephalopods became very important in subsequent 

 periods, and the evolution of the class will be dwelt upon in suc- 

 ceeding chapters. 



Arthropods. — Among the Arthropods the simpler forms 

 (Crustaceans) only are known from the Cambrian. Trilobites are 

 by far the most abundant and significant Cambrian Arthropods 

 (Figs. 39-40). In fact they are the most important Cambrian fos- 

 sils. We have seen that the threefold subdivision of the Cambrian 

 system is based upon the changes in the Trilobite fauna. Examples 

 of the most characteristic Cambrian genera are shown in Fig. 39. 



