122 THE ORIGIN AND EVOLUTION OF LIFE 



acteristic of Cambrian and Ordovician times, is closely similar 

 to that of Lingiila anatina, a species living to-day. Represen- 

 tatives of the genus Lingula {Lingulella) have persisted from 

 Cambrian to Recent times. The great antiquity of the brachi- 

 opods as a group is well illustrated by the persistence of Lingula 

 (Cambrian — Ordovician — Recent), on the one hand, and of 

 Terehratula (Devonian — Recent), belonging to a widely differ- 

 ing family, on the other. These lamp-shells are thus charac- 

 teristic of all geologic ages, including the present. Reaching 

 their maximum radiation during the Ordovician and Silurian, 

 they gradually lost their importance during the Devonian and 

 Permian, and at the present time have dwindled into a rela- 

 tively insignificant group, members of which range from the 

 oceanic shore-line to the deep-sea or abyssal habitat. 



By the Middle Cambrian the continental seas covered the 

 whole region of the present Cordilleras of the Pacific coast. 

 In the present region of Mount Stephen, B. C, in the unusually 

 favorable marine oily shales of the Burgess formation, the 

 remarkable evolution of invertebrate life prior to Cambrian 

 time has been revealed through Walcott's epoch-making dis- 

 coveries between 1909 and 1912.^ It is at once evident (Figs. 

 20-27) that the seashore and pelagic life of this time exhibits 

 types as widely divergent as those which now occur among 

 the aquatic Invertebrata; in other words, the extremes of 

 invertebrate evolution in the seas were reached some thirty 

 million years ago. Not only are the characteristic external 

 features of these soft-bodied invertebrates evident in the fossil 

 remains, but in some cases (Fig. 22) even the internal organs 

 show through the imprint of the transparent integument. 

 Walcott's researches on this superb series have brought out 

 two important points: First, the great antiquity of the chief 



1 Walcott, Charles D., 1911, 1912. 



