]\Iaryland Geological Survey 595 



the culmination of Spirifcr iiicsastrialis and S. vicsacostalis. Douvillina 

 cayuta, also a species which in New York is most abundant in the earlier 

 Chemung sediments and, as has been observed, attains remarkable pro- 

 fiTsion and variety in the Maryland strata, is only an occasional associate 

 of the species in New York. Generally speaking, shells of the form 

 represented in fig. 8 arc of the most common expression in Maryland and 

 New York, short wings with margins broadly rounded to the median 

 sinns. The extension of cardinal angles shown in figs. 10 and 11, is alsO' of 

 frequent occuiTence; fig. 1, pi. Ivii, represents an extreme condition. A 

 noteAvorthy expression is shown by the shells found in green shale on Polish 

 Mountain where some are short hinged and some long hinged (fig. 10) 

 but all li.ave the margins of the median sinus not rounded but quite 

 strongly elevated, a feature persistent irrespective of variation in form. 

 This expression is also seen in some specimens from other localities. 



The careful study of the variations of this plastic species may eventu- 

 ally disclose the significance of these expressions, whether of geographical 

 or zonal value. The usefulness of such an undertaking will be further 

 shown in determining how far the later expressions of the species pass 

 into those species of recognized carboniferous shells hearing similar 

 features, of which S. ncwherri/i. imbrex and marionensis may be cited 

 among the American forms. Until this latter task is done it remains 

 unsafe to carry the Devonian line upward to whatever extent the species 

 may seem to run. 



Length 20 mm. ; width 40 mm. 



This species is characteristic of and coextensive with the Chemung 

 of Maryland. It is found at nearly all exposures of that member. Some 

 of the observed horizons are recorded in the following paragraph. 



Occur mice. — Jennings Formation, Chemung Member. Allegany 

 Grove, 13G3, 3325, 2340, 2355, 2380 abundant, 2800, 2960 abundant, 

 2980 abundant, 3000 abundant; Ellerslie, Pennsylvania, 1316 abundant, 

 1474 abundant; Williams Road, Polish Mountain, 1967, 2382, 2708, 

 2725; National Eoad, on Polish Mountain, 2016, 2185, 2949; 2 miles 

 north of mouth of Town Creek, 1949 ; Town Creek, 2122 abundant, 2228 

 abundant, 249(; alnmdant, 3982 abundant, 4480 abundant, 4562, 



