714 GEOLOGICAL SURVEY OF IOWA. 



SURFACE marked by sharp radiating stride, with broader 

 spaces between, which are crossed by fine undulating striae, 

 and, towards the margins, by strong imbricating lines of 

 growth which give a thickened condition to that part of the 

 shell. 



This species resembles, in many of its characters, the O. umbraculum, but differs 

 from all the figures and descriptions which have come under my observation : it may 

 be compared with O. undiferiis of SCHNUR ( BUNKER and VON METER'S Palaeonto- 

 graphica, pa. 217, pi. 45, f. 1). 



Fig. 5 a. View of the ventral valve, with the greater part of the shell exfoliated. 

 Fig. 5 b. View of the dorsal valve, from which the striae are nearly obliterated. 

 Fig. 5 c. Profile view of the same. 



Fig. 5 d. Enlargement of the surface, showing the concentric striae. [ Incorrect shading 

 has represented the depressed portions as elevations.] 



Geological formation and locality. Lower Coal measures : St. Glair coun- 

 ty, Illinois. 



Terebratula subtilita. 



Terebratula subtilita : HALL in Stansbury's Report of an Exploration of the Valley of the 



Great Salt Lake of Utah, 1852. 



This species has a very wide geographical distribution; being known in Eastern 

 Ohio, Indiana, Illinois, Iowa, Missouri, Kansas, Nebraska, the Pecos villages in 

 New-Mexico, and in several other places along the range of the Rocky mountains. 

 After a careful examination of many specimens, I find the punctate structure very 

 feebly represented under an ordinary magnifier. The exfoliation, however, often takes 

 place in parallel laminae, and the surfaces present a striato-punctate appearance. 



It is possible that this may prove to belong to the Genus ATHTRIS, some species 

 of which it resembles in form and surface striae. 



