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Kirtley F. Mather 
are convex until just below the ears where they are more or 
less concave, the auriculations being sub-equal but somewhat 
variable in outline in different individuals; the surface orna- 
mentations consist of concentric costae, six or eight in number, 
with concentric striae occupying the space between them, about 
six to eight striae between each two costae; the markings are 
more closely spaced near the ears and spread out below, are much 
fainter on the umbo than toward the margins, and in crossing 
the ears are bent outward. The concentric costae are not nearly 
so prominent nor so numerous as those in the typical A.? inter- 
lineatus. 
Horizon and locality. Brentwood limestone : northeast of 
Fayetteville, Arkansas (Station 135). 
Aviculopecten arkansanus n. sp. 
Plate XV, figure 13. 
Description. Shell small, obliquely sub-oval in outline ex- 
clusive of the ears, length and height sub-equal, hinge-line about 
three-fourths as long as greatest length of valve below, the 
latter being along a line which if produced would meet the 
hinge-line 15 mm. or so in front of the beak ; left valve convex, 
the beak full and separated from the auriculations by a sinus 
on either side, that in front being the stronger of the two which 
converge upward at an angle of about 65°, the beak directed 
obliquely forward and extending slightly beyond the hinge-line; 
ears comparatively large and sub-equal, each separated from 
the margins of the valve below by a byssal sinus, that in front 
deep and strongly concave, the posterior one shallow and gently 
concave, outline of anterior ear rounding narrowly to the car- 
dinal margin while that of posterior ear meets the hinge-line 
in an acute angle ; surface of valve ornamented by about 15 fine, 
radiating, ridge-like costae separated by fiat striations about 
three times their width, in the center of which near the ventral 
margin faint intermediate costae may sometimes be detected, 
concentric lines not apparent in the material at hand, auricu- 
lations unsculptored except for a single, faint, broad, undefined 
radial depression across the anterior ear. 
Horizon and locality. Hale formation: East Mountain, Fay- 
etteville, Arkansas (Station 136). Kessler limestone: East 
Mountain, Fayetteville, Arkansas (Station 209). 
