332 PALAEONTOLOGY OF OHIO. 



between tlie beak and central region; beak small, depressed, and but 

 slightly projecting beyond the cardinal margin, near the middle of which 

 it is placed with scarcely perceptible obliquity ; surface showing fine, 

 obscure lines, and a few stronger marks of growth, with faint traces of 

 radiating striae, and an entirely distinct set of regular, transverse, waved 

 or arched, parallel, little linear ridges or costse, which appear to have 

 resulted from the markings of the surface of the object upon which the 

 shell had grown; lower (right?) valve unknown. 



Ventro-dorsal and transverse diameters each about 0.95 inch ; convex- 

 ity of the upper (left ?) valve, about 0.13 inch. 



The shells of this type seem to vary so much in form that it is barely 

 possible that this may be only an extreme variety of Placunopsis carbon- 

 aria, Meek and Worthen. (See Illinois Geol. Rep., V., pi. 27, figs. 2a-d.) 

 Its much longer and straighter hinge, however, and angular lateral ex- 

 tremities, with its less prominent, more nearly central beaks, and nearly 

 obsolete radiating striae, all combine to give it so different an appearance 

 that with such means of comparison as we now have on hand I can but 

 regard it as a distinct species. 



It is not possible, from the specimens yet examined, to be absolutely 

 sure that these Carboniferous shells agree exactly with the genus Placu- 

 nopsis, but so far as can be determined they seem to agree quite closely 

 with it. The Illinois specimens show the lower valve to be flat, or at 

 least to conform to the contour of the surface to which it seems to have 

 grown by its whole under surface. It shows no traces of a perforation 

 or sinus for the passage of a byssal plug, as in Anomia, while the upper 

 valve, which, from the direction of its slight obliquity, appears to be 

 the left one, always shows a curious mingling of concentric and radi- 

 ating markings, with an entirely different set of regular, transverse, or 

 oblique lines or ridges, as if the latter had been produced by the mark- 

 ings of some other shell upon which it had grown, as we often see in 

 Anomia, Crania and some other attached shells. 



Baron de Reyckholt has described a genus, Anomianella, from the Car- 

 boniferous rocks of Tournay, that had a similar habit of growing on other 

 shells, and assuming their style of markings. At one time I thought it 

 highly probable, from a notice I had seen of his genus, that it might 

 include our shells of the type under consideration; but on seeing the 

 figures of "his A. proteus, which is supposed to be the typical species (I 

 have not yet seen his description), I find it to be an oval shell, without 

 the slightest traces of a straightened cardinal edge or marginal beak. 

 Consequently it is very improbable that our shells can properly be placed 



