The Rogers Gap Fauna of C en trad Kentucky 135 



Gap, and at 59.8, 59.7, 59.5, 55.0, 54^.6, and 54.3. It belongs to 

 the group of species in which the upper margin, anterior to the 

 beak, descends rather rapidly below the level of the cardinal out- 

 line posterior to the beak, rounding strongly into the anterior out- 

 line, which extends only 5 or 6 mm. in front of the beak in shells 

 DO mm. in length. Shells larger than this in size are rare. The 

 basal margin usually makes an angle of about 22 degrees with the 

 cardinal outline. It rounds into the posterior margin. The latter 

 meets the cardinal outline obliquely at an average angle of about 

 45 degrees, provided the general convexity of this part of the 

 posterior margin be ignored. The umbonal ridge is only moder- 

 ately angular toward the beak, and posteriorly it merges into the 

 general convexity of the shell. Anteriorly there is a weakly 

 defined mesial sulcus, sometimes almost obsolete, which leaves the 

 basal margin of the shell practically straight or slightly convex. 

 The anterior muscular scar is well defined. 



Compared with Modiolopsis ohlonga, Ulrich, from the strata 

 below the Fulton layer at Covington, opposite Cincinnati, the basal 

 margin is more divergent from the cardinal outline ; the posterior 

 outline is less truncate and more rounded, so that the point of 

 greatest curvature is farther above the basal outline ; and the junc- 

 tion of the cardinal and posterior margins, instead of being sub- 

 angular, is more nearly rounded. [Moreover, compared with the 

 height, the shell is relatively shorter. While Modiolopsis rogers- 

 ensis evidently is closely related to M. ohlonga, it is sufficiently 

 distinct for purposes of discrimination. The type of Modiolopsis 

 oblonga is quite strongly and evenly convex, with no well defined 

 umbonal ridge. 



26 — Orthodesma cf. suhnasutnm, Meek and Worthen (Plate 

 I. Fig. 11) — A small lamellibranch, 34 mm. in length, and nearly 

 13 mm. in height posteriorly, was found at the railroad cut 59.1 

 miles from Ludlow. It bears a general resemblance to Orthodesma 

 suhnasutnm, from the Trenton of Illinois, in form and in the pres- 

 ence of low, broad, concentric undulations, but the anterior part 

 is less depressed below the cardinal margin. The umbonal ridge 

 is distinct toward the beak, merging into the general convexity 



31 



