SPECIES FROM THE HYDRAULIC LIMESTONES OF THE LOWER HELDERBERG GROUP
Annals of the New York Academy of Sciences
Shell small to minute, the largest individuals yot observed not exceeding five-eiglitlis of an inch in greatest diameter, while the most of those observed are not more than two-thirds as great. Valves depressed convex, or, more commonly, appearing very Bat, as 8een on the surface of the stone. Iiingeline straight, nearly as long a8 the width of the shell below, and the latter usually more than the length, frequently nearly once and a half as great. Ventral valve characterized by a very narrow
... by a very narrow and nearly vertical cardinal area, and a iisually more or less twisted or otherwise distorted beak. Dorsal valve slightly more convex than the ventral, with a perceptible mesial depression extending froin beak to base, becoming liroad and undefined below the middle of the length. Surface of the shell marked by coarse and somewhat rigid radiating strile, which are distinctly alternating in size ; the principal ones proportionally very strong. The small size of the shell, with the strong radiating nnd alternate striae, are distinguishing features of the species. There is n o species resembling it, to any depree, among the fossils of New York rocks of a corresponding age. I t presents much more tlie featllrcs of forms of the genus from the Coal Measures than any heretofore descrihed from Silurian roclts of hncrica, and will not be readily confounded with a.ny known species. Formation and Locality.--Tn the hydraulic beds OF the Lower IIclderberg group, at Belleville, Saridusky County, and at Green-Contributions to Invertebrate Palieontology. 509 field, Ohio ; amociated with JZeristelln bella, hTucleospira rotundata and Leperditia aka, occurring sonietimes in great numbers, almost covering the s.ufaccs of slabs. Genus SPIRIFERA Sowerby. Spiritera Vanaxemi. PLATE V, figs. 4 and 5.