DESCRIPTIONS OF SPECIES. 409 



The relations of CI. o)-Mcidaris a,nd CI. sublmneUosa are of the closest. I find 

 that the characters pointed out by McChesney to distinguish them can not be 

 generally relied upon, and only hold good in selected cases. The only distinction 

 which I have been able to observe in the material examined is that the Pennsylvanian 

 species seems to be, as a rule, somewhat more transverse than the older form, but I 

 doubt if to this difference alone could be given specific significance, even if it proves 

 to be a constant one. I am, therefore, prepared to see McChesnej^'s name orhioularis 

 succeed to Hall's svhlojmellosa for this species. 



Under the name of Spirigera jylanosiilcata White has already cited this .species 

 from Colorado, from Rush (Rock) Creek, in Lake County. I am not sure that the 

 material upon which this identification rests has come into mj^ hands, but I have it 

 from other areas in the shape of two small, not very perfect shells, each from a sepa- 

 rate station. The lai'ger of these is less than S mm. in diameter, and therefore some- 

 what below the average size of Mississippi Valley specimens, but otherwise I see no 

 characters by which to distinguish them. 



McChesney's original description of CI. oriimda/ris, to which access is usualljr 

 difficult, 1 have cited below in full. 



"Athyrus orbicularis (n. s.). 



"Shell subcircular or varying from transverseljr subelliptical to subovately 

 elongate, moderately gibbou.s. Dorsal valve less than the ventral, depressed convex, 

 rarely slightly elevated in front; beak clo.sing the foramen of the opposite valve. 

 Ventral valve convex, strongly gibbous toward the beak; front sometimes flattened, 

 but without true sinus; beak prominent, slightly incurved, and vertically or some- 

 what obliquely truncated by a circular foramen. 



"Surface distinctly marked b}' concentric lamellose bands of growth. 



"This species is closely allied to A. siMamellom of Hall (Geol. Rejj. Iowa, 

 p. 702), from which it is easily distinguished by having a lai'ger ventral than dorsal 

 valve, while in that the dorsal valve is much the largest, and by the much greater 

 elevation or thickening of the ventral valve toward the beak. 



"It has sometimes been referred to Tei'ebratula 'plcmo-sulcata of Phillips, from 

 which it difi^ei-s in being a less rotund shell, and in the much less gibbosity or infla- 

 tion of its venti-al valve toward the front of the shell, as well as in other respects. 



"Geological position and localities: In Coal Measures, particularly the upper 

 portion, extensively distributed in the Western States." 



Locality. — Grand River region, Glenwood Springs (station 2193a). 



DIELASMA King, 1850. 

 DiELASMA BoviDENS Morton?. 



PL VII, figs. 11,11a. 

 1836. Terehratula hovidcns. Morton, Am. Jour. Sci., (1), vol. 29, p. 1,50, pi. 2, fig. 4. 



CoalMeasure.s: Ohio Valley. 

 1856. Terehratvln miikpunctaia . Hall, Pacific Railroad Rept., vol. 3, p. 101, pi. 2, figs. 1, 2. 



r'arbonit'erous: Pecos Village, N. Mex. 



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