344 



HISTORICAL GEOLOGY 



sented in a subordinate role in the Cambrian fauna, while 

 others seem to have made their appearance after the close of 

 the period. Of these none is more important 

 than the graptolites (Figs. 346 and 347), 

 those colonies of little polyps strung on 

 stems. Being freely float- 

 ing animals they were 

 easily transported by ocean 

 currents, and hence single 

 species had an almost 

 world-wide range. Their 

 relatives, the corals, here 

 became important for the 

 first time. It is to be 

 noted that in the early 

 stages of their evolution 

 the corals were represented 

 chiefly by the solitary 

 hornlike forms (Fig. 348), 

 whereas the habit of living in compact colo- 

 nies (Fig. 349) became more prevalent in 

 later periods, until to-day the compound 

 corals far outnumber the solitary varieties. 

 The mollusks (p. 298), of which only the pteropods and 

 cap-shaped gastropods had been noteworthy in the Cambrian, 



FIG. 346. A colony 

 of graptolites. Each 

 little tooth on the 

 blades held an indi- 

 vidual polyp. The 

 central portion may 

 have served partly 

 as a float. 



FIG. 345. Head 

 of a very sim- 

 ple crinoid of 

 the Ordovician 

 period. 



FIG. 347. A branching colony of graptolites impressed upon a piece of shale. 



