THE PRIMORDIAL, OR CAMBRIAN AGE. 



53 



foot of naturalist lias yet trod to scan its productions. 

 Very different estimates have been formed of tlie 

 amount of life in this period, according to tlio position 

 given to its latest limit. Taking some of the more 

 modern views of this subject, we might have included 

 among the Primordial animals many additional crea- 

 tures, which we prefer noticing in the Silurian, since 

 it may at least be aflBrmed that their head-quarters 

 were in that age, even if they had a beginning in the 

 Primordial. It may be interesting here, however, to 

 note the actual amount of life known to us in this 

 period, taken in its largest scope. In doing this, 

 I shall take advantage of an interesting table given by 

 Dr. Bigsby,* and representing the state of knowledge 

 in 1868, and shall group the species in such a manner 

 as to indicate the relative abundance of distinct types 

 of structure. We find then — 



Plants (all, or nearly all, supposed to be 

 seaweeds, and some, probably, mere 

 tracks or trails of animals) 



Sponges, and similar creatures , 



Corals and their allies 



Starfishes and their allies 



Worms 



Trilobites and other crustaceans . 



Lamp- shells and other molluscoids 



Common bivalve mollusks 



Common univalve mollusks and their 

 allies 



Higher mollusks, nautili,cuttle-fishes,etc 



In all 

 * "Thesaurus Siluricus." 



22 species. 



27 



4 



29 

 442 

 193 



12 



172 



65 



972 



