PALEONTOLOGY OF IOWA. 677 



concentric striae and somewhat stronger radiating striae, 

 which are more conspicuous than the others, and, towards 

 the margin, by stronger imbricating lamellose lines of growth. 



This species bears so close a resemblance to S 1 . keokuk, that I cannot find characters 

 for a specific distinction. The plications are usually less numerous in a shell of the 

 same size; but it is somewhat variable in form, and particularly in the extension of 

 the cardinal extremities and the gibbosity of the shell. 



Fig. 4 a. A ventral valve, showing the extended cardinal extremities. 



Fig. 4 b. Cardinal view of the same. 



Fig. 4 d. Dorsal view of a more gibbous specimen, with the cardinal extremities rounded. 



Geological position and locality. In the Brecciated or St. Louis limestone 

 at the mouth of Lizard creek, "Webster county, Iowa. 



FOSSILS OF THE KASKASKIA LIMESTONE. 



As previously stated in the Geological Report, this lime- 

 stone is not known within the State of Iowa ; thinning out 

 in its northern prolongation before reaching so far in that 

 direction. In order therefore to illustrate the entire series of 

 Carboniferous limestones, I have found it necessary to go 

 beyond the limits of the State for the materials. 



The Crinoideae, as well as some of the Brachiopoda, have 

 been furnished by Mr. WORTHEN from his extensive cabinet. 

 Having examined the rock in some of its best developed 

 localities, I am prepared to recognize it as very distinct in 

 its palaeozoic features from either of the preceding limestones, 

 not having thus far found any species which are common 

 to this rock and either of those already described. 



