CIvASSlFlCATlON CAMBRIAN BRACHIOPODA — WAI^COTT I4I 



DEVELOPMENT IN CAMBRIAN TIME 



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

 taining the Olenellus or Lower Cambrian fauna. That such existed 

 in pre-Cambrian time seems almost certain when the advanced stage 

 of development of some of the earliest known forms is considered. 



In the preceding diagram the known occurrence of the families of 

 brachiopoda in strata of Cambrian age is graphically shown. The 

 diagram is based on the data contained in tables prepared for the 

 monograph giving a summary by families. The Obolidge, with 7 gen- 

 €ra, 9 subgenera, 183 species, and 17 varieties, has the greatest devel- 

 opment, and the family continues into the base of the Ordovician 

 with 2 genera, 7 subgenera, and 36 species. The Acrotretidae has 

 6 genera, i subgenus, 93 species, and 19 varieties, with the greatest 

 development in the Middle Cambrian and with a smaller representa- 

 tion in the Lower Ordovician. The Billingsellidae, with 9 genera, 

 2 subgenera, 95 species, and 12 varieties, has a strong develop- 

 ment in the upper Middle Cambrian and passes into the Ordovician, 

 where it disappears. The three families mentioned include about 48 

 per cent of the genera, 80 per cent of the subgenera, 81 per cent of 

 the species, and 81 per cent of the varieties included in the Cambrian 

 brachiopoda. The development of genera of the remaining families 

 containing 3 genera or more is as follows: Paterinidat, 4; Obolel- 

 lidae, 4; Siphonotretidse, 6; Syntrophiidas, 3 ; or 17 genera of the 24 

 outside of the Obolidge, Acrotretidae, and Billingsellidae, which con- 

 tain 23 genera. The remaining 7 families include 7 genera. 



Of the 46 genera from the Cambrian, 20 occur in the Lower 

 Cambrian, 31 in the Middle Cambrian, and 23 in the Upper Cambrian. 



SCHEME OF CLASSIFICATION 



In order that we may have a graphic illustration to aid in de- 

 scription, the following table is inserted. The ordinal classification 

 of Beecher [1891], with emendations, is taken as the basis for the 

 orders, while the arrangement of superfamilies is practically that of 

 Schuchert [1897], with such emendations and additions as greater 

 information has rendered necessary. Dr. Charles Schuchert has 

 been most helpful in discussion and criticism of this scheme of classi- 

 fication, and I am also indebted to Mr. E. O. Ulrich for a discussion 

 of the classification of the Protremata. Due acknowledgment will 

 Idc made in the monograph to many persons who have aided in 

 various ways in making the monograph much more complete and 

 itseful than it otherwise would have been. 



