NO. 4 CAMBRIAN AND PRE-CAMBRIAN AT HELENA 295 



6. Acrotretaci. sagittalis (Salter) (pi. 3, fig. 10, of Rothpletz). 

 This is a Middle Cambrian and Upper Cambrian species of Europe 



and a Middle Cambrian species in the Atlantic Coast provinces of 

 North America. The specimen figured by Rothpletz is a compressed 

 ventral valve and is much more likely to be Acrotreta dcprcssa (Wal- 

 cott) ^ of the Aliddle Cambrian of the Rocky Mountains. 



7. Kutorgina cf. perrugata Walcott (pi. 3, fig. i, of Rothpletz). 

 This is a figure of an undeterminable fragment of a large 



brachiopod. 



8. Hyolifhcs cf. billiiigsi Walcott (pi. 3, figs. 2 and 4, of Roth- 

 pletz). 



This is a very common form of Ilyolithcs and is abundant in the 

 Park and Wolsey shales at Helena and elsewhere. 



The fragments of trilobites figured by Rothpletz on plate 3 have 

 no special significance except that they are of Cambrian age. 



COMMENT ON PALEONTOLOGICAL EVIDENCE 



Rothpletz does not seem to have established a strong case for the 

 Lower Cambrian age of the Park shale fauna. There does not appear 

 to be the slightest foundation for assuming that because there are 

 some superficial resemblances in form between the distorted shells in 

 the shale and the various species with which he has identified them, 

 the fauna is, therefore,* of Lower Cambrian age. The fossils are 

 unlike the Lower Cambrian species with which they have been iden- 

 tified, and such a known association of species does not occur at any 

 one locality and zone anywhere in the world. 



The impression made upon me is that Rothpletz once having been 

 misled by his interpretation of the stratigraphy was unconsciously 

 influenced to determine his fossils as of Lower Cambrian age. The 

 purpose for which he went to Helena, that is, to discover pre-Cambrian 

 fossils in the Belt formations," would have been served by Middle 

 Cambrian fossils quite as well, provided they came from what had 

 previously been identified as a pre-Cambrian formation. The dis- 

 covery of fossils in a Middle Cambrian shale at Helena in which they 

 had previously been found is evidently not what Rothpletz thought he 

 was doing either in the field or in the laboratory. 



Monogr. U. S. Geol. Surv., Vol. 51, 1912, pi. 66, figs. 8, 8a-c. 

 Rothpletz, p. 8. 



