484 GEOLOGY OF OHIO. 



space of an eighth of an inch on the middle of a shell, being finer within and 

 coarser beyond that point. On the partially exfoliated shell, fine radiating vascular 

 lines are perceptible. Ventral valve flat, discoidal, circular in outline, or perceptibly 

 elongated in some cases ; the apex a little more than one-third the length of the 

 shell from the posterior margin. Foramen small, elongate-elliptical, narrow, not 

 extending more than one-fourth of the distance from the apex toward the margin, 

 and the depression somewhat further. Surface marked as in the other valve. 



This shell would appear to be identical with the one described and 

 figured by Messrs. Meek and Worthen as D. nitida ? under the supposi- 

 tion that it was the same as that figured by Prof. Phillips, in the Geol. 

 Yorkshire Coast, vol. ii, pi. 11, figs. 10-13; but it differs very much in 

 outline from those figures, as well as those given by other authors, in its 

 circular form ; those being ovate, narrowed behind and widened in front ; 

 also, in having the apex much more distant from the margin. They also 

 cite D. Missoitriensis Shumard, as a synonym of the European species. 

 That author indicates his shell as parabolic in outline; from which state- 

 ment I should consider it as distinct from the present species. 



Formation and Locality. — In the Coal Measures at Carbon Hill and 

 Flint Ridge, Ohio; also in Illinois and Iowa. 



Genus CRANIA- Retzius. 



Crania carbonaria. 



Tlate XI, figs 11 and 12. 



Crania carbonaria Whitf, Ann. N. Y. Acad. Sci., 1882, p. 229. 



Shell small, none of the specimens observed exceeding three-eighths of an 

 inch in diameter; subcircular in outline, or varied in form by the outline of the 

 object to which they are attached. Free valve depressed convex., marked by a few 

 concentric lines of growth; attached valve thin, but with a slightly thickened mar- 

 gin. Posterior muscular impressions large and submarginal, the others being nearly 

 central and forming a small elevation just posterior to the middle of the valve. 



The shells of this species are found attached to the spines of Zea- 

 crinus and other bodies, one of those figured being upon the operculum 

 of Naticopsis. The}* are ver} ; thin, and not easily detected in the rough- 

 ened condition caused by the adhering material in which most of the 

 fossils from these beds are found. Species of this genus are rather rare 

 in the Coal Measures, but very few having been described. Crania 

 permiana Shumard, from the white limestones of the Guadalupe Mts., 

 Texas, is a large form, and probably not a Crania, according to the de- 

 scription given. C. modcsta White and St. John, from the Coal Measures 

 of Iowa, is described as "rather small, finely punctate, smooth, except 

 somewhat strong concentric lines of growth toward the margins. Upper 

 valve moderately convex, umbo oblique, nearly central. L,ower valve 

 moderately concave." There would appear to be some similaritj* be- 

 tween the upper valves of this and the Ohio species; but the remark 

 concerning the lower valve being ''moderately concave" throws consid- 



