248 ANNUAL EEPORT SMITHSONIAN INSTITUTION^ 1915. 



CAMBRIAN ANNELIDS." 



I had often searched the fine shales of the pre-Cambrian and 

 Cambrian strata for remains of annelids, but it was not until the 

 summer of 1910 that anything more than trails and borings were 

 found. 



The annelids of the Burgess shale, like the holothurians and 

 medusae, are pressed so flat that the worm is represented by onlj?^ a 

 thin film. Fortunately this is darker than the shale and usually 

 shiny, and the contents of the animal are often preserved as a glis- 

 tening silvery surface, even to the fine details of structure. How 

 clearly the specimens exhibit both external and internal characters 

 is shown by the plate figures (pi. 12), wdiich are reproduced from 

 photographs made by reflected light. 



RELATIONS TO LIVING ANNELIDS. 



The discovery of this remarkable group of annelids in the Burgess 

 shale member of the Stephen formation opened up a new point of 

 view on the development of the Annulata. The fact that from one 

 very limited locality there were collected 11 genera belonging 

 to widely separated families points clearly to the conclusion that 

 the fundamental characters of all the classes had been developed 

 prior to Middle Cambrian time. 



CAMBRIAN BRACHIOPODA.=' 



The chief characters of the Cambrian brachiopod are illustrated 

 on plate 13. It may be studied in three ways — historically, geologi- 

 cally, and zoologically. 



The conditions in which the Cambrian brachiopods are found 

 indicate that some of them were gregarious in habit, and that many 

 persisted through marked changes of environment and sedimenta- 

 tion. One species,^ for instance, is found in sandstone, siliceous 

 and argillaceous shale, and limestone. It has a wide distribution in 

 the Cordilleran province of western North America and has a verti- 

 cal range in the layers of rock of 2,000 feet or more. Other forms, 

 such as Micromitra haydeni, are known only from one locality and 

 one layer of rock. A large number of species occur in sandstone 

 and shales that are evidently of shallow-water origin; others occur 

 in limestones that were probably deposited in relatively deep water. 

 The evidence indicates that their habitat largely ranged from be- 

 tween tides to a depth of 1.000 to 2,000 feet. Some forms may have 



1 Middle Cambrian Annulnta, Smithsonian Misc. Coll., vol. 57. no. .5, 1!>11. Annelid, 

 from (iniihis, rini; ; applied to worms. 



2 Walcott, Monoj;r. .51, U. S. Geol. .Survey, 2 vols., 1912. 

 "Micromitra (IpMdella) pannula Walcott. 



