MARYLAND GEOLOGICAL SURVEY 259 



LEPTAENA GIBBOSA (James) 

 Plate LIV, Fig. 25 



Strophomena gibbosa James, 1874, Cincinnati Quart. Jour. Sci., vol. i, p. 333. 

 Leptaena gibbosa Foerste, 1909, Bull. Sci. Lab. Denison Univ., vol. xiv, 



p. 316. 

 Leptaena gibbosa Foerste, 1912, Idem., vol. xvii, p. 116, pi. i, figs. 5a-c. 



Description. " Shell fragile, semi-oval ; cardinal line extended to or 

 a little beyond the width of the shell farther forward, deflected at the 

 extremities ; lateral and front margins regularly rounded. Ventral valve 

 slightly convex in the umbonal region, hut at about one-third or one-half 

 the distance from the beak, toward the front and lateral margins, it 

 curves suddenly upward, then rounds off, and is deflected as suddenly the 

 other way to the front and sides, forming a high rounded ridge, giving 

 to the shell a decidedly gibbous form ; this hump extends to about l/8th 

 of an inch of the cardinal line on each side, where the shell is rather 

 depressed from the umbonal slopes outwards to the deflected extremities 

 immediately in front of the cardinal line; cardinal area linear; beak 

 rather prominent, projecting, minutely perforated; six to eight slight 

 wrinkles on the umbonal region. Surface covered by fine radiating striae, 

 increased by interstitial additions, somewhat variable in size on the front 

 slope, but quite uniform on the umbone and to the lateral margins; 

 crossed by fine concentric striae. Interior not observed. 



" Dorsal valve (exterior) gently concave to about the middle, where it 

 makes a sudden curve, conforming to the shape of the other valve; the 

 two valves are so closely drawn together as to leave scarcely any visceral 

 space; beak very little elevated above the cardinal line; area no more 

 than a rather sharp edge of the hinge ; radiating striae, as far as observed 

 same as on the ventral valve. Interior nearly flat, or slightly convex to 

 the base of the ridge, in front and laterally to within about one-quarter 

 or one-eighth of an inch of the cardinal line, where there is a flat depres- 

 sion extending to the lateral margins ; the curve to the front from the top 

 of this ridge is abrupt, corresponding to the exterior; cardinal process 

 bifid, erect, rather prominent, curving slightly anteriorly, crenulated 



