THE CAMBRIAN PERIOD. 



287 



plants. The whole plume appears to have floated free in the sea. 

 They were scantily preserved in the earliest Cambrian (Fig. 124, e), but 

 quite abundantly toward its close. The secret of their preservation 

 probably lies in the fact that, being floating forms, they most often 

 settled in quiet and rather deep waters off-shore, where very fine silts 

 accumulated, and where, therefore, the conditions were favorable for 

 burial without destructive action. 



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 



Fig. 124. — Cambrian Ccelenterata; supposed corals, medusae, and graptolites. a and 

 b, Archceocyathus rensselcericus Ford, a problematic fossil referred by some paleon- 

 tologists to sponges, and by others to corals; c and d, Brooksella alternata Wal- 

 cott, 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, Phyllo- 

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



in Fig. 124, c and d. Their impressions are found in the lower division 

 of the Cambrian. 



Obscure forms of corals were also present (Fig. 124, a and b) though 

 none of the higher types has yet been discovered. The forms found 

 resemble sponges so much that they were long regarded as such, but 

 recent microscopical examination by Hinde seems to show that they had 

 a coralline nature. 



True sponges were present in some abundance and ranged throughout 

 the period. Their spicules have been sometimes found where the 

 general form and the fibrous parts have perished. 



The Cambrian Protozoa. — It is probable that many of the low 

 simple forms classed as protozoans were present, but owing to their 

 unsuitability to preservation as fossils, only a few identifiable forms 

 have been found. 1 



1 Several are described by G. F. Matthew in "The Protelenus Fauna," Trans. N. Y. 

 Acad. Sci., Vol. XIV, 1895,' pp. 101-153. 



