THE PRIMORDIAL, OR CAMBRIAN AGE. 53 



foot of naturalist has yet trod to scan its productions. 

 Very different estimates have been formed of the 

 amount of life in this period, according to the 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 affirmed 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) . . 22 species. 



Sponges, and similar creatures . 27 



Corals and their allies .... 6 



Starfishes and their allies . . 4 



Worms 29 



Trilobites and other crustaceans . . 442 



Lamp- shells and other molluscoids . 193 



Common bivalve mollusks . . 12 

 Common univalve mollusks and their 



allies 172 



Higher mollusks, nautili,cuttle-fishes,etc. 65 



In all . . . 972 

 * "Thesaurus Siluricus." 



