462 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



demata, an association which also occurs in the Middle Cambrian St. Croix 

 sandstone of Wisconsin. Numerous heads occur, but only one fragmentary 

 pygidium has been seen. An illustration of the pygidium is taken from a 

 specimen found in the St. Croix sandstone 4 miles southeast of Lake City, 

 Minnesota. 



Formation and locality: Middle Cambrian, Flathead formation, north 

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

 Butte creeks, Yellowstone National Park. 



PTYCHOPARIA (?) DIADEMATA Hall Sp. 

 PI. LXIV, figs. 2, 2a-c. 



Conocephalites diadematus Hall, 1SG3: Sixteenth Ann. Eept. Nevr York State Cab. 

 Nat. Hist., p. 107, PI. VII, figs. 36-38; PI. VIII, figs IS, 21. 



This species is associated with Ptychoparia (L.) wisconsensis. The cen- 

 tral portions of the head are quite abundant and associated with the sepa- 

 rated free cheeks. Two finely preserved hypostomas and one imperfect 

 pygidium also occur on the slabs of limestone. The pygidium is illustrated 

 by a specimen from the St. Croix sandstone, found in the bluff near Hudson, 

 Wisconsin. 



Formation and locality: Middle Cambrian, Flathead formation, north 

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

 Butte creeks, Yellowstone National Park. 



ARIONELLUS Barrand. 



Arionellus LEVIS n. sp. 

 PI. LXV, fig. 1. 



It is with little doubt that the specimen illustrated is referred to this 

 genus. It has a strongly convex, minute head, 4 mm. in length; glabella 

 nearly as broad as long, narrowing slightly toward the broadly rounded 

 front; glabella furrows barely visible as four short, slightly depressed lines. 

 It is separated from the fixed cheeks and frontal limb by a narrow groove, 

 which is all that breaks the general convexity from the frontal margin back 

 to the occipital furrow. The occipital furrow is deeply impressed between 

 the glabella and the very narrow depressed occipital ring. Fixed cheeks 

 slightly convex, sloping abruptly toward the facial sutures; anteriorly they 



