DESCEIPTIONS OF SPECIES. 407 



2229, 2231, 2233, 2236, 2237, 2239, 2240, 2242, 2243, 2246, 2247, 2248, 2249, 2284, 2309, 

 2323, 2331, 2332, 2337, 2340, 2341, 2342, 2349); Molas formation, lower, middle, and 

 upper portions of the Hermosa formation, and in the Rico formation; Ouray (sta- 

 tions 2195, 2195a, 2195b); Hermosa formation. Crested Butte district (stations 

 2244, 2245, 2280, 2291, 2293, 2297, 2298, 2299, 2303, 2305, 2306, 2318); Weber lime- 

 stone and Maroon formation. Leadville district (stations 2252, 2254, 2259, 2261, 

 2262, 2263, 2264 ?, 2267, 2273, 2274, 2275); lower, middle, and upper portions of the 

 Weber formation, and in the Robinson limestone. Grand River region (station 

 2324), Glenwood Springs (stations 2193a, 2329). 



CLEIOTHYRIS King, 1850. 

 Cleiothyris orbicularis McChesney. 



1860. AthyriM orbicularis. McChesney, Desc. New Pal. Foss., p. 47. 



Coal Measures: Western States. 

 ? 1860. Spirigera missouriensis. Swallow, Acad. Scl. St. Louis, Trans., vol. 1, p. 650. 



Coal Measures: Montgomery and Chariton counties, Mo. 

 1866. Alhyris 2)lanosulcata. Geinitz, Carb.. und Dyas in Nebraska, p. 42. 



Coal Measures: Omaha City, and Plattsmouth, Nebr. 



1874. Alhyris sublamellom. Derby, Cornell Univ., (Science) Bull., vol. 1, No. 2, p. 10, pi. 2, figs. 



9-12; pi. 3, figs. 15, 21, 29; pi. 6, fig. 16; pi. 9, figs. 5, 6. 

 Coal Measures: Bomjardim and Itaituba, Brazil. 



1875. Spirigera planomlcata. White, U. S. Geog. Geol. Surv. W. 100th Mer., Kept., vol. 4, p. 143, pi. 



10, figs. 5a-d. ( Whole volume published in 1877. ) 

 Carboniferous: Santa Fe, N. Mex., Rush Creek, Lake County, Colo. 

 1897. Cleiothyris orbicularis. Schuchert, U. S. Geol. Surv., Bull. No. 87, p. 182. 

 ? 1897. Cleiothyris missouriensis. Schuchert, U. S. Geol. Surv., Bull. No. 87, p. 182. 



In its representation in American strata the genus CleiotJiyris is restricted, so 

 far as itnown, to the Carboniferous period. Throughout the existence of this type 

 it follows few lines of variation, but within certain limits and in a few particulars 

 these shells are very variable. The most obvious character is that of size, and it is 

 at the same time the least significant. 



A number of specific names have been proposed for American representatives of 

 the genus, but experience shows that it would be difficult to establish the validity 

 of man}^ of them if reference were had only to the uniformity and persistence of 

 differential characters. 



In the Upper Carboniferous representatives of the Oieiothyris group are usually 

 small of size and in numbers rare. The first specific name proposed for a 

 Pennsjdvanian Cleiothyris is Athyris orbic%ilaris McChesney, which was published 

 early in 1860. In the same year, though probably subsequent to the publication of 



