New LameUibranchiata. — Ulrich. 
281 
Shell small, transversely subovate, or Aviculoid in outline, 
alate and much the widest posteriorly. Valves appressed, 
with point of greatest convexity above the center. Cardinal 
margin straight, its length equalling two-thirds of the greatest 
length of the shell. Anterior end forming nearly aright angle 
with the hinge line, then rounding gently into the basal mar- 
gin, the curve of the edge being very nearly uniform from the 
antero-cardinal angle to the postero-basal region. Here the 
margin rounds quickly up into the posterior edge, the curve 
becoming gradually less to the angular junction with the hinge 
line, which it meets at an angle of about 115°. Beaks small, 
projecting very little above the hinge line, and situated about 
one-fourth of its length from the anterior end of the shell. 
Umbonal ridge inconspicious, only appreciable because of a 
slight concavity in the cardinal slope. Surface with faint con- 
centric lines of growth. Anterior muscular impression, as seen 
in casts of the interior, circular and small, but clearly defined. 
The largest specimen seen has the following dimensions : 
Length of hinge line, 12 mm ; length from antero-cardinal an- 
gle to postero-basal margin (greatest length of shell) 17mm; 
greatest hight, from center of basal margin to posterior ex- 
tremity of hinge line, 13 mm; greatest convexity of two valves 
4.5 mm. 
Hall's M. aviculoides is a narrower and more convex shell 
The species also resembles M. truncata Hall and M, parva of 
this paper. The first, however, is much larger, has a wider an- 
terior end and a thicker shell. The second also has a thicker 
shell and is much more convex. 
Position and locality : Rather rare in yellowish shales of the Cincin- 
nati group, near the tops of the hills about Cincinnati, Ohio. 
Modiolopsis parva, u. sp. 
