4 i6 HISTORICAL GEOLOGY 



Graptolites. — This group will be discussed more fully in the next 

 chapter (p. 427), since it reached its greatest abundance and develop- 

 ment in the Ordovician. The word graptolite (Greek, graptos, written, 

 and lithos, a stone) is descriptive, since, when 

 preserved in shale, as is usually the case, grapto- 

 lites have the appearance of lead-pencil marks 

 (Fig. 397, p. 428) with saw teeth on one or both 

 sides. Graptolites were slender organisms, plant- 

 like in appearance, usually resembling the modern 

 hydroids. As is true of hydroids, they were com- 



Fig. 387. — Cam- posite animals in which the individuals lived in 

 brian coral: Archceo- n 1 1 • 1 c 1 1 



cyatkus rensselaricus. Cells StrUn S on one ° r both s, f « of a Render, 

 horny axis which united the " colony." The 



form of these colonies varied greatly, as will be shown later (p. 428). 



Graptolites appear for the first time in the Upper Cambrian. 



Jellyfish. — The discovery of fossil jellyfish in Cambrian rocks is 

 most surprising, since these animals have no bony skeletons or shells. 

 Specimens preserving the external form as well as something of the 

 interior structure have been found. They must have been buried in 

 mud soon after they died, for otherwise they would have been de- 

 stroyed by the worms and predatory crustaceans associated with them. 



Sponges lived in some abundance in portions of the Cambrian and 

 were represented by several genera. They are known by their sili- 

 ceous spicules, which were either embedded in horny fibers or inter- 

 laced into a supporting framework, and which were preserved because 

 of their resistant character. 



Protozoa 



Theoretically there is every reason to believe that the simple 

 unicellular protozoa were as abundant in the Cambrian seas as in the 

 present oceans, but no fossils have been discovered which are known 

 with certainty to belong to this group. 



Summary 

 Evolution during the Cambrian. — It has been seen that at the 

 beginning of the Cambrian many classes of animals were already 

 in existence, and that the advanced stage of development of some of 

 them, notably the trilobites, taken in connection with the traces of 

 Pre-Cambrian life, indicates that life was well-advanced before 

 Cambrian time began. Whether or not the evolution of this Pre- 

 Cambrian life was rapid is not known. Evolution during the Cam- 



