THE LOWER SILURIAN ROCKS. 647 



and terminal, projecting well forward though but little above the hinge. 

 For a short distance behind the lower part of the thin projecting margin 

 of the byssal opening the shell is compressed. A wide, undefined sulcus 

 crosses the valves from the sinus in the posterior margin to the posterior 

 side of the beaks. Surface of the best specimen seen marked with about 

 twenty simple costae. These increase in size from the cardinal margin 

 downward, the first four being much smaller than those beneath them, 

 while anterior to the tenth, which may be the largest, a gradual decrease 

 is observable. The concentric growth lines at e very regular and unusually 

 distinct. 



The large costae will distinguish this fine species from all of the pre- 

 ceding forms of this genus. The next species, O. ampla, though closely 

 related, is readily distinguished by several obvious differences. O. 

 notabilis also is closely allied but has differently shaped ends and seems 

 to be without radiating plications on the cardinal slope. 



Formation a?id locality: The type specimen was collected by Prof. 

 Edward Orton in the upper beds of the Cincinnati group at some locality 

 in Warren county, Ohio. Two imperfect specimens belong to the cabinet 

 of Prof. J. M. Safford and were collected by him in Hickman county, 

 Tennessee, where they occurred in strata regarded as equivalent to the 

 middle or upper beds of the Cincinnati group of Ohio. 



Opisthoptera ampla, n. sp. 



Plate 47, Fig. 7. 



The specimen upon which this species is founded is a cast of the in- 

 terior and not a good one either. Still it preserves sufficient of its 

 specific characters to give us a fair idea of the position of the species in 

 the genus. Evidently it is more closely related to O. laticostata than to 

 any other known. Yet, the greater height, especially of the anterior part, 

 more oblique and more deeply sinnate posterior margin, larger ventral 

 part and differently arranged radiating costae, are differences obvious 

 enough to render the separation of good specimens of the two species a 

 matter of small difficulty. The wing also is broader and the anterior end 

 is not compressed, while the byssal opening seems to have been larger. 

 Being a cast the costae near the cardinal border are difficult to make out, 

 but enough of them can be determined on the wing to show that there is 

 not that uniformity in the increase of the size of the costae which per- 

 tains to O. laticostata. The large plications will of course distinguish 

 the species at once from all the other forms of the genus save O. nota- 

 bilis, and that one is very different in nearly every other respect. 



Formation and locality: Middle beds of the Cincinnati group, Cin- 

 cinnati, Ohio. 



