Fauna of the Morrow Group 
211 
A right valve from the Hale formation, although not as 
strongly convex as the more mature individual figured by 
McChesney, agrees so closely with other specimens from the 
Danville locality that there can be no doubt that it belongs to 
this species. The fine threadlike concentric striae are ordinarily 
regular but near the margins two or three may be crowded 
together and form faint concentric ridges across the surface 
of the shell. The shell itself is comparatively thick, as is com- 
mon in this genus. 
Horizon and locality. Hale formation: East Mountain, Fay- 
etteville, Arkansas (Station 137). 
Nucula kessleriana n. sp. 
Plate XV, figures 22, 22a. 
Description. Shell small, transversely oval, length to height 
about as seven to four, convexity moderate ; anterio-dorsal mar- 
gin broadly convex, rounding abruptly into the convex ventral 
margin with the anterior extremity narrower and slightly more 
produced than the posterior; posterio-dorsal margin gently con- 
cave near the beak and rounding broadly to the ventral margin ; 
beak full and prominent, moderately elevated above the hinge- 
line, situated a little behind the middle and directed very slightly 
posteriorly; hinge plate represented in the internal cast by a 
deep groove beneath the beak with the impressions of at least 
six close-set teeth; surface of cast smooth; surface of shell not 
ornamented and marked only by faint concentric growth lines. 
Dimensions of the type specimen are : length, 14.7 mm. ; height, 
8.8 mm. ; convexity of left valve, about 3.3 mm. 
Remarks. This species, based upon a left valve preserved as 
both an internal cast and external mold, is referred to Nucula 
rather than to Yoldia, in the absence of information as to the 
pallial line, because the valves do not gape. It is unlike all other 
Carboniferous species of these genera, distinguished from those 
forms with a similar shape by its lack of sculpturing and from 
the smooth forms by its shape. 
Horizon and locality. Kessler limestone : East Mountain, Fay- 
etteville, Arkansas (Station 209). 
