320 CAMBRIAN BRACHIOPODA. 



The earliest representatives of the Nisusiinse are Nisusia and its subgenus JameseTla; both 

 occur in the upper portion of the Lower Cambrian. Nisusia is unknown later than the 

 upper portion of the Middle Cambrian, and Jamesella is unknown above the base of the Upper 

 Cambrian. ProtortMs, along with its subgenus Lo-peria, is first known in the central portion 

 of the Middle Cambrian; it disappears toward the close of the Upper Cambrian. Billingsella 

 is known from the central Lower Cambrian and on to nearly the close of the Upper Cambrian. 



Of the genera of the Billingsellidse other than Billingsella, Wynnia occurs in the Middle 

 Cambrian. Orusia appears in the upper portion of the Middle Cambrian and continues well 

 up into the Upper Cambrian, and Otusia is found in the upper part of the Middle Cambrian. 

 None of these genera appear to have had descendants in Ordovician time. Wimanella, 

 although a more primitive form than Billingsella, is not known to occur as low down in the 

 Lower Cambrian, and its upper range is only to the upper portion of the Middle Cambrian. 

 It is derived from the Billingsella radicle. 



Eoorihis is first knowai from the lower portion of the Middle Cambrian and continues 

 through the Upper Cambrian into the Ordovician. Its line of evolution through Billingsella 

 appears to have been from the same radicle as Nisusia, which occurs in the upper portion of 

 the Lower Cambrian. The genus Finltelnburgia is derived from Eoortliis in the lower portion 

 of the Upper Cambrian. The Ordovician genus OrtMs is assumed to have developed from 

 Eoorihis in late Cambrian time, and Eostrophomena in the earl}^ Upper Cambrian. 



Sijntrophia, beginning in the lower Middle Cambrian, continues into the Ordovician, and 

 appears to have descendants in Clarkella and in Huenella. 



Swantonia suggests the Rhynchonellidse in form, but it has a spondylium and it majr have 

 been the progenitor of both SijntropMa and ProtortMs. It is represented in the diagram as 

 an offshoot from the radicle of Nisusia, without descendants. A review of the characters of 

 the various subgenera of Orthis from the Ordovician strata leads us to expect to find repre- 

 sentatives of DinortJiis in the Cambrian. According to Hall and Clarke [1892a, p. 195] this 

 form is characterized by having the ventral valve depressed so that it is flat or concave 

 over the pallial region, in this respect resembling Hehertella [Hall and Clarke, 1892a, p. 198]. 

 Among Cambrian forms ProtortMs (Loperia) dugaldensis (PI. XCIX, figs. 5, 5a-h) of the 

 Middle Cambrian has the depressed ventral valve, but otherwise it differs from DinortMs in 

 having a spondylium in the ventral valve and in the absence of a cardinal process in the dorsal 

 valve. Thus far there appear to be no representatives of OrtMs, Plectorihis, OrtliostropMa, 

 PlatystropMa, HeterortMs, Bilohites, Dalmanella, or other Ordovician or Silurian orthoids 

 among the Cambrian species. In endeavoring to trace a genetic connection between the 

 Cambrian Billingsellidse and the Orthidse of the Ordovician we are met at once with the great 

 hiatus caused by lack of material for study and comparison from the formations between the 

 Upper Cambrian and the Middle Ordovician. The Orthidse of the Middle Ordovician include 

 a large group of brachiopods, differing radically from the Billingsellidse in shell structure, 

 whose ancestral line in the Lower Ordovician and Upper Cambrian is unknown. That this 

 break will be filled is highly jsrobable, but meantime the more or less conjectural lines of 

 descent of various authors must be carefully reviewed in each case in connection with all the 

 stratigraphic and paleontologic evidence afforded b}'' all known Lower Ordovician formations, 

 especially in the American and European areas. Systematic studies now in progress by 

 members of the United States Geological Survej'' may afford much valuable information that 

 will be of service in this connection. 



