316 



CAMBRIAN BRACHIOPODA. 



of the Obolidffi, Acrotretidse, and Billingsellida?, which contaiu 21 genera. The remaming 

 7 famihes inchide 7 genera. 



Of the 44 genera from the Cambrian, 19 occur in the Lower Cambrian, 31 in. the Middle 

 Cambrian, and 20 in the Upper Cambrian. 



FiGUKE 17.— Diagram illustrating known distribution of families in Cambrian strata. * 



EVOLUTION OF THE CAMBRIAN BRACHIOPODA. 



SCHEMATIC DIAGRAM. 



In order to formulate, so far as possible, in a graphic manner my conception of the evolution 

 and lines of descent of the Cambrian Brachiopoda, gained by the investigations described in 

 this monograph, a schematic diagram (fig. 18) has been prepared. 



This diagram is necessarily tentative and incomplete, but it mil serve to point out my 

 present conceptions of the lines of evolution of the various genera, and it shows clearly the very 

 rapid development of the primitive atrematous genera in early Cambrian time. 



The genera identified from each of the three divisions of the Cambrian may be ascertained by 

 glancing at the columns headed Lower, Middle, and Upper Cambrian for the generic and 

 subgeneric names mcluded in each column. 



The most primitive but not the oldest species known to me from the Cambrian formations 

 is Rustella edsoni (PI. I, figs. 1, la-e). This is found in association with Olenellus thomjysoni 

 in Vermont, where it occurs at nearly the summit of the Lower Cambrian, and not low down 

 toward the base of the Cambrian system. Probably the oldest-knoA\'n brachiopod, or the species 

 lowest in the stratigraphic series, is Trematobolus excelsis (PI. LXXXII, figs. 8, 8a-d). This is 

 far advanced m development and must have had a long line of descent from the protegulum 

 stage. Another verj' ancient type is Micromitra. A species with ornamented surface (M. 

 {IpTiidella) louise) occurs 2,850 feet down in the Lower Cambrian of the Canadian Rockies and 

 Micromitra (Iphidella) pannula (PL IV) is abundant in the upper portion of the Lower Cambrian. 

 Micromitra (Paterlna) labradorica (PI. II) is found deep in the Lower Cambrian of New Brunswick 



