362 



CAMBRIAN PERIOD 



plentiful throughout the system. The early forms are chiefly of 

 the low conical type, while more amply coiled and spiral forms be- 

 came common later. Some of them resemble modern gastropods 

 closely. 



Sea worms (Vermes, p. 686) left evidence of their abundance by 

 borings, tracks, etc. (Fig. 326). A few cystoids, the forerunners of 

 the beautiful crinoids (stone lilies), represented the echinoderms. 



Ccelenterates were represented by graptolites, medusae and 

 polyps (corals). The eccentric freaks of fossilization are nowhere 

 better illustrated than here. Relics of graptolites, among the most 

 delicate of animal forms, and of medusa (jelly-fish), among the soft- 

 est of animals, were preserved, while some stronger types left scant 

 record of themselves. Graptolites, now extinct, were slender, plume- 



a d 



Fig. 327. CAMBRIAN CCELENTERATA : supposed corals, medusae, and grapto- 

 lites. a and b, Archaeocyathus rensselcericus Ford, a problematic fossil referred by 

 some paleontologists to sponges, and by others to corals; c and d, Brooksclla alternata 

 Walcott, supposed casts of the gastric cavities of medusae; c, a supposed exumbrella 

 in which the interumbrella lobes are a prominent feature; d, a view of a supposed 

 umbrella with six lobes and a depression over the central stomach; e, Phyllograptus 

 (?) cambrensis Walcott, the hydrosoma of a graptolite. 



like organisms (e, Fig. 327), consisting of a series of hard cells, in 

 which the individual zooids lived, attached to a common slender 

 axis. The whole colony appears to have floated free in the sea. 

 The secret of their preservation probably lies in the fact that, being 

 floating forms, they settled in quiet waters off-shore, where fine silts 

 accumulated, and where the conditions were favorable for burial 

 without destruction. The most singular case of fossilization is 

 the preservation of traces of jelly-fish, or at least of what are so 

 identified (Fig. 327, c and d} in the Lower Cambrian. Obscure 

 fossils of corals are found (Fig. 327, a and b), the forms of which re- 

 semble sponges so much that they long were regarded as such. 

 Corals seem to have been more abundant in some other parts of the 

 world than in North America. 



