556 Contributions to Invertebrate Palaeontology. 



in number on diflferent individuals but usually ten or twelve ; those making 

 the centre of the valve and on the urahonal slope being nearly straight in their 

 direction from the beak to the base of the shell ; while those nearer the ante- 

 rior end become more and more curved in their direction as they approach the 

 margin. From three to five intermediate smaller costse occupy the usually 

 slightly concave but often flattened interspaces. The entire surface of the 

 valve is often marked by more or less strongly marked concentric lines of 

 growth, which in crossing the stronger radii often form lamellose projections 

 on their surfaces when perfectly preserved, but are usually represented by 

 small knotty prominences as commonly seen. The posterior wing is often 

 marked by indistinct radiating lines, tliough not uncommonly tliese are entirely 

 obsolete, and the concentric lines are strongly marked. The right valve of 

 this shell is very slightly concave, proportionally smaller than the left with 

 the radiating lines much subdued, and the concentric lines not so elevated or 

 knotty. 



The specimens of this species observed from the rocks above the 

 " Bone-bed," in Ohio, have been left valves mostly. I have a recol- 

 lection, however, of having seen one slab in the Ohio State Collec- 

 tion, at Columbus, which contained the impression of one right and 

 one left valve, possessing the usual features of the species common 

 in the Hamilton rocks of New York; and which was said to be 

 from layers above the "Bone-bed." The specimen figured on 

 Plate XI, fig. 17, is from this horizon, and presents all the features 

 common to the New York Hamilton forms, including the great 

 gibbosity of the left valve. In the Upper Helderberg limestones 

 of New York few representatives of this species have been recog- 

 nized, and those present a coarser ruder form than the Hamilton 

 group specimens ; and specimens from the limestones from below the 

 "Bone-bed," near Columbus are not uncommon, but are very 

 large, very coarse, and rude in character, having but a distant re- 

 semblance to the typical forms of the species. These I strongly 

 suspect are properly a distinct species, but the examples thus far 

 obtained have been of so imperfect a character as not to furnish 

 characters sufficiently marked to determine this question. 



Formation and Locality. — In rocks above the " Bone-bed ;" hori- 

 zon known as the " Petroleum Rock," in Tully Township, Marion 

 Co., Ohio. The specimen figured is from the State collection at 

 Columbus, and was collected )>y the Rev. Mr. Herzer. 



