EVIDENCES OF PRIMITIVE LIFE WALCOTT. 249 



had a greater bathymetric range, but the evidence in favor of such 

 a range is not knoAvn to me. A table of the species in the monograph 

 showed that with few exceptions each of the species is confined to 

 one type of sediment. 



More than .500 species and varieties of Cambrian brachiopods were 

 studied and between 40 and 50 of Ordovician. Of the Cambrian 

 forms, 10 genera, 2 subgenera, 21 species, and 1 variety persisted 

 into the Ordovician. 



Approximately 1,0.50 different localities bearing brachiopods were 

 examined, and the same genera were found often to exist in widely 

 separated regions, as, for example, a clear relationship was shown 

 between brachiopods from the Scandinavian Peninsula and those of 

 eastern Canadian localities, Avhile in many other instances those of 

 the western Cordilleran region of North America were related to 

 those from China. 



MICROSCOPIC STRUCTURE. 



One important deduction from microscopic examination of the 

 shells was the differentiation of certain genera and species from the 

 Cambrian and Ordovician hitherto classed together,^ the microscopic 

 shell structure of one being of granular material pierced by small 

 pores, and in the other of fibrous material. On the other hand, the 

 microscopic structure of two other orders- in question is so similar 

 that an unbroken line of descent is indicated. 



We do not know of any brachiopods in strata older than that con- 

 taining Lower Cambrian fauna. Yet when the advanced stage of 

 development of some of the earliest-known forms is considered it 

 seems almost certain that such existed far back in pre-Cambrian time. 



THEORETICAL EVOLUTION OF CAMBRIAN CRUSTACEA FROM THE 



BRANCHIOPODA.^ 



The Cambrian crustacean fauna suggests that five main lines or 

 stems (Branchiopoda, Malacostraca, Ostracoda, Trilobita, and Mero- 

 stomata) were in existence at the beginning of Cambrian time, and 

 all of them had already had their inception in Lipalian time or the 

 period of pre-Cambrian marine sedimentation, of which no known 

 part is present on the existing continents. Examples of some of these 

 forms are shown in plates 9, 10, 14, and 17. 



In the accompanying diagram (p. 250) the attempt is made to show 

 the relations of Cambrian crustaceans to a theoretical ancestral stock, 



1 Cambrian Billingsellidse and Ordovidan Protremata. 



2 Cambrian and later Pentameracea. 



2 Walcott : Middle Cambrian Branchiopoda, Malacostraca, Trilobita, and Merostomata, 

 Smithsonian Misc. Coll., vol. 57, no. 6, 1912. 



