500 



GEOLOGY 



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. These are illustrated 

 in Fig. 376, c and d. Their impressions are found in the Lower 

 Cambrian. Obscure fossils of corals are present (Fig. 376, a and 6), 

 the forms of which resemble sponges so much that they were long 



Fig. 376. Cambrian Ccelenterata: supposed corals, medusae, and grapto- 

 lites. a and 6, Archceocyathus rensselcericus Ford, a problematic fossil 

 referred by some paleontologists to sponges, and by others to corals; 

 c and d, Brooksella alternata Walcott, supposed casts of the gastric 

 cavities of medusa?; c, a supposed exumbrella in which the interum- 

 brella lobes are a prominent feature; d, a view of a supposed umbrella 

 with six lobes and a depression over the central stomach; e, Phytto- 

 graptus (?} cambrensis Walcott, the hydrosoma of a graptolite. 



regarded as such. Corals seem to have been more abundant in 

 some other parts of the world than in North America. 1 



Lower types. Sponges were present in some abundance through- 

 out the period. It is probable that many of the low, simple forms 

 classed as protozoans existed, but only a few identifiable forms 

 have been found. 



Implied life. The existence of so much animal life implies 

 much vegetable life to supply the necessary food, as heretofore 

 noted. Furthermore, various characteristics of the fossils suggest 

 the presence of animals not known from fossils. A large percentage 

 of the known Cambrian animals were provided with shells, tests, 

 plates, or other forms of hard coverings. In the main, these 

 appear to have been protective devices, and imply enemies or com- 

 bative rivals against which protection was needed. Perhaps the 



1 Howchin, Quar. Jour. Geol. Soc. Vol. LXIV, p. 237. 



