CAMBRIAN FOSSILS. 453 



ditions very fine concentric striae can be seen, and there are also usually 

 present more or less distinctly marked lines of growth. 



The generic character of this species has not been fully ascertained, 

 but the material from the Park and specimens from the typical locality 

 at Red Wing, Minnesota, lead me to think that this can not be referred to 

 the genus Billingsella. It appears to be an Orthis of the Plectorthis group 

 of Hall and Clarke. 



A comparison with specimens of Orthis sandbergi from the typical 

 locality at Red Wing, Minnesota, shows the two shells to be specifically 

 identical, as far as the comparison of casts in sandstone can be made with 

 well-preserved shells on the surface of a limestone slab. This is the only 

 species of the type known to me in the Cambrian fauna. It is a type that 

 is developed in the Ordovician fauna, and I think it will be found to occur 

 in the Calciferous-Chazy fauna of New York and the St. Lawrence Valley. 



Formation and locality: Upper Cambrian, north side of Elk Pass, 

 between Buffalo and Slough creeks, Yellowstone National Park. 



PLATYCERAS Conrad. 



Platygeras primordialis Hall? 



Pi. LXIII, fig. 1. 



Platyceras primordialis Hall, 1S63, Sixteenth Ann. Eept. New York State Cab. Nat. 

 Hist., p. 136, PL VI, fig. 28. 



A single species of Platyceras occurs on a slab of limestone in asso- 

 ciation with Billingsella pepina, Ptychoparia (L.) tvisconsensis, and Ptyclto- 

 paria (?) diadematus. 



So far as can be determined from a comparison of the single specimen, 

 it is probable that the forms are identical. 



Formation and locality: Middle Cambrian, Granatin terrane, north side 

 of Soda Butte Creek, below saddle on ridge between Pebble Creek and 

 Soda Butte Creek, Yellowstone National Park. 



