644 GEOLOGY OF OHIO. 



The outline and general expression of this shell is almost exactly the 

 same as in O. casei M. and W. The anterior side seems to be somewhat 

 straighter, a little more oblique, and more obtuse, but the principal differ- 

 ence no doubt lies in the radiating costas. These are small, subequal, 

 and rather closely arranged in O. casei, but in O.fissicosta the greater 

 part of them, though at first of nearly equal size, soon become separated 

 by gradually widening interspaces in which new ones are produced by 

 bifurcation and interpolation. Bundles of three to five are then formed 

 with the central one the strongest and most prominent. The inequality 

 of the costae, however, becomes less in nearing the margin of old exam- 

 ples, while in the cardinal region they appear alwaj^s to be more equal 

 than in the central portion of the valves. 



Formation and locality: Meek gives Cincinnati, Ohio, as the locality 

 for his specimen, but this is probably an error. So far as can be learned 

 the species occurs only in the upper beds of the Cincinnati Group, in 

 which I have seen it at Clarksville and Waynesville, Ohio. Good speci- 

 mens are very rare. 



Opisthoptera alternata, n. sp. 



Plate 49, Figs. 9 and 11 ; also cut on page 645 



In this species the surface markings seem to be almost exactly as in 

 O.fissicosta, the costse being at first simple, close together and equal, 

 then, by widening the interspaces and the addition of new ones, either 

 by interpolation or bifurcation, an alternation of the costse is produced 

 that is highly characteristic of the two species. Though agreeing so 

 closely in this respect, a considerable difference in the outline is at once 

 apparent. Thus, in O. alternata the posterior wing is much shorter and 

 the posterior margin therefore much less oblique — indeed, it may some- 

 times be almost vertical — the angle formed with the hinge line in O.fis- 

 sicosta being less than 40° and over 70° in O. alternata. The anterior 

 outline on the other hand is more oblique in the latter. Among other 

 differences it will be found that there is no broad, fold in the wing of O. 

 alternata as is always present in O.fissicosta and O. casei, while the high 

 anterior part of the valves is rounded instead of subangular, the latter 

 being the case in those species. The ligamental area, furthermore, is 

 much more of a feature, being nearly as wide and distinct as in O. obliqua 

 (see plate 48, figure 6). In the last species the form is somewhat similar, 

 yet they may be distinguished at once by the greater convexity and the 

 almost fiat anterior side of O. obliqua. In O. extenuata the anterior side 

 is also flattened, but a more striking distinction is afforded by the remark- 

 able posterior prolongation of the wing. 



Formation and locality: Upper beds of the Cincinnati Group, 

 Waynesville, Ohio. It seems to be a rare shell. 



