CARBONIFEROUS PERIOD. 131 



Surface marked by a few obscure radiating- strice, wliich are most con- 

 spicuous near the median line of each valve ; fine concentric striae are nu- 

 merous, and in the adult shell tliere are also strong imbricating linos of 

 growth near the front and lateral margins. 



Length, fifteen millimeters ; breadth, foiu-teen millimeters ; heig'ht, 

 twelve millimeters. 



This shell is not only without the plications so common to the paleozoic 

 species of BhynclioneUa, but it has also a different general aspect. This has 

 caused some doubts to arise as to the propriety of referring it to that genus, 

 but the broken ventral beak shows no other characters than those of 

 BhyncJionella, and the shell-structu.re is also distinctly fibrous and not 

 punctate. 



Position and locality. — Strata of the Carboniferous period ; Rock Canon, , 

 Wasatch Range, near Provo, Utah, where only a single example was 

 obtained. 



Rhynchonella Rockymontana Marcou. 



Plate IX, fig. 1 a, 1>, c, and d. 



Tcrebratula Roclcymontana Marcou, 1858, Geology of North America, 50. 

 Ehynchonella eatoniaformis McCbesuey, ISCO, Description New Pal. Fossils, 49. 



Shell rather large, inflated, subtrihedral in outline, broadest near the 

 front ; sides somewhat regularly rounded from the antero-lateral portions 

 to the beaks. Ventral valve having its greatest convexity towai'd the beak ; 

 sides sloping away from the middle with slight convexity and becoming 

 flattened or sometimes even a little concave near the lateral margins ; beak 

 rather sniall, prominent, and closely incurved over that of the other valve ; 

 mesial sinus very broad but not deep, prolonged far upward at the front, 

 becoming obsolete about the middle of the valve and is entirely wanting 

 upon its posterior portion ; from two to four depressed angular plications 

 occupy the mesial sinus and disappear with it, the sides and posterior por- 

 tion being free from plications. Dorsal valve more capacious than the 

 ventral ; mesial fold distinct at the front, and, like the mesial sinus, becoming" 

 obsolete about the middle of the valve ; from three to five plications like 

 those of the other valve mark the fold, but the surface upon each side of it 

 is plain like that of the posterior portions of both valves. Tlie whole 



