428 



HISTORICAL GEOLOGY 



Graptolites began in the Cambrian as small, bushy forms (Fig. 398 A) which, as a 

 rule, lived throughout life attached to the sea bottom. Before the close of this period 

 however, a change in the mode of life occurred which was to give the class entirely dif- 

 ferent habits and as a result bring about important modifications in the structure. 

 For some unknown reason, perhaps to avoid a new creeping enemy, the colonies left 

 the sea bottom. At first the branches of the bush-like colonies hung suspended, 



Fig. 397. — Graptolites: A and B, Didymograptus nitidus; C, PhyllograptuS ' typus , 

 D, Monograptus clintonensis ; E, Goniograptus postremus ; F, Diplograptus prislis, 

 C, Phyllograptus angustifolius ; H, Dichograptus octobrachiatus ; /, Dictyonema flabelli- 

 forme; J, Climacograptus bicornis ; K, Tetragrapius fruticosus. 



later they became horizontal, and still later the branches were turned upward. This 

 change was accompanied by a reduction in the number of branches. The irregular 

 many-branched early forms gave way to regular, many-branched colonies (Bryo- 

 gfaptus, Fig. 398 li), then to eight-branched (Dichograptus, Figs. 398 C and 397 H), 

 these, in turn, to four-branched forms (Tetragraptus, Figs. 398 D and 397 K), 

 and these to two-branched forms (Didymograptus, Figs. 397 A and 398 E). In 



