NO. 4 CAMBRIAN AND PRE-CAMBRIAN AT HELENA 293 



All of the specimens in the shale are more or less flattened by com- 

 pression and usually more or less distorted. Frequently the distor- 

 tion of the brachiopods takes the forms shown by Rothpletz, plate 2, 

 figures 4 to 12; plate 3, figures 3 and 5. When the dorsal valves of 

 Lingulella hclcna are shortened by pressure they are ai)t to arch up 

 and curve down rapidly on the posterior side ; the usually rounded 

 postero-lateral margins also assume a more angular outline. 



DETERMINATION OF GENERA AND SPECIES BY ROTIII'LETZ, AND NOTES 

 BY WALCOTT 



/. Prostospongia cf. fcncstrata Salter (pi. 2, figs. 2-7, of Roth- 

 pletz.) 



This form of sponge spicule is met with in both the Lower and 

 Middle Cambrian formations. It is particularly abundant in the 

 shales of the Middle Cambrian. Specific and even generic determina- 

 tion is very difficult. 



2. Riistclla cdsoni var. pcnttv^onalis Rothpletz (pi. i. fig. 10, of 

 Rothpletz). 



This is evidently a dorsal valve of Obolits {IVestonia) ella, which 

 preserves the concentric striation so characteristic of that species 

 when the outer surface has been removed by clinging to the matrix. 

 Rustella edsoni, with which it is identified, has been found only in 

 association with Olenellus on Lake Champlain, Vermont, and near 

 York, Pennsylvania, on the eastern side of the continent. Obolns 

 ( IVestonia) cUa is a common form in the Park shale on Mount 

 Helena, and I am surprised that Rothpletz did not find it there and 

 also more abundantly where he collected the fauna southeast of 

 Helena. 



For comparison I am illustrating (pi. 44, figs. 6, 7, 8, and 9) dorsal 

 valves of O. {W.) ella from the Park shale on the southeast slope of 

 Mount Helena, also ventral valves which have been shortened and 

 broadened a little by compression. Obolus (IVestonia) ella occurs in 

 passage beds between the Lower and Middle Cambrian.^ It is most 

 abundant in the Middle Cambrian, and rarely if not doubtfully present 

 in the Upper Cambrian. 



J. Lingulella hclena (Walcott) (pi. 2, figs. 1 and 2. of Rothpletz). 



This is very abundant in the Park shale on the southeast slope of 

 Mount Helena, also in the same band of shales where outcrops occur 



^ The stratigraphic position of the beds containing this fauna in northern 

 Montana is open to discussion. I have long considered them as doubtfully 

 Lower Cambrian or Middle Cambrian. They may be claimed for either until 

 more detailed studies are made at the typical localities where they occur. 



