LII-I-; 



361 



Moll us ks (p. 686) were well represented, Cephalopods (chambered 

 shells), the highest class of mollusks and are found in the upper- 

 most beds of the Cambrian. As they were even then highly devel- 

 oped, there is little doubt that the class had passed through a long 

 history before the end of the period. Pelecypods (bivalves, oysters, 

 clams, etc., b, Fig. 325) lived throughout the period, though their 



I%- 325. CAMHKIAN MOLLUSKS: a, Hyolilliis <innriinii.\- Hillings, a Lower 

 C';iml>ri;m pteropod; /', I-'onJilla troycnsis Barrandc, a Lower Cambrian peleeypod; 

 r, Stfiiotlurii riisi>x<i Hall, a i apulid gastropod of the Lower Cambrian; d, T roc us 

 .\<irtil<>x<->isis Walcott, a gastropod with well-developed spire; c, Platyccras primavum 

 Hillings, a Lower Cambrian gastropod;/, Op/iilcla primordalis Winchell, an Upper 

 Cambrian gastropod. 



fossils are not abundant. Like brachiopods, pelecypods are bi- 

 valves, but unlike the brachiopods, the valves are not bilaterally 

 symmetrical. Gastropods (univalves, c, d, e, Fig. 325) are rather 





T 



L 





Fig. 326. CAMBRIAN VERMES: borings and trails, o, a surface of sandstone 

 showing annelid borings, with mounds of sand heaped about their mouths and with 

 trails leading away from some of them. 



