58 NORTH AMERICAN INDEX FOSSILS. 



42. C. nicoUetti Clarke. (Fig. 1258.) Ordovicic. 

 Large, often slightly curved; annulations sharp, separated by 



broad, concave interspaces; septa profoundly concave. 

 Stones River of Minnesota; Bighorn of Wyoming. 



43. C. randolphense Worthen. (Fig. 1259.) Mississippic. 

 Slightly compressed, rather rapidly tapering; annulations low, 



round, with shallow interspaces. 



Chester group of Illinois; Mississippic of Nevada. 



XII. Dawsonoceras Hyatt.^ 



Differs from Cycloceras in having prominent, frilled or wrinkled 

 growth lines between the annulations. Siluric-Devonic. 



44. D. annulatum Sowerby. (Figs. 1260, 1261.) Siluric. 

 Annulations moderately strong, slightly nearer the older suture ; 



siphuncle nearly centren. Interspaces asymmetrical. 

 Niagaran of Indiana, Wisconsin, etc. 



44a. Van americanum Foord. Siluric. 



With stronger annulations midway between the sutures; inter- 

 spaces symmetrically concave. 



Widely distributed in the Niagaran of North America. Also in 

 the Upper Monroe of Michigan and Canada. 



XIII. Protokionoceras* Grabau and Shimer. 



Non-annulated orthoceracones, with the surface marked by fine 

 and generally rather close longitudinal striae. Siphuncle more or 

 less moniliform. Genotype P. medullar e (113.11). Siluric-Devonic. 



45. P. meduUare (Hall). (Fig. 1262.) Siluric. 

 Large, tapering; sutures widely separated (by nearly half the 



diameter in the older part) ; surface striae fine, alternating and 

 close together. (Type of genus.) 



Niagaran of Wisconsin, Indiana and Canada (Guelph), and 

 doubtfully New York (Guelph), 



46. P. crebescens (Hall). (Fig. 1263.) Siluric. 

 Differs from the preceding in the shorter camerse, large, strongly 



nummulitic, centren siphuncle, and distant, rather broad longi- 

 tudinal striae. 



Niagaran (Guelph) of Wisconsin, Canada and New York. 



