276 PALEOZOIC PALEONTOLOGY. 
STROPHEODONTA PLANULATA Hall. 
Plate XXVII., Figs. 1-2. 
1859. Stropheodonta planulata Hall, Pal. N. Y., vol. II1., p. 184, pl. 
16, figs. 9-12. 
Description.—Shell plano-convex, subsemi-elliptical in outline, wider 
than long, the hinge-line equal to, a little greater or a little less than 
the width of the shell below. Pedicle valve depressed-convex, slightly 
flattened toward the cardinal extremities, the beak not prominent, 
cardinal area narrow, linear. Brachial valve flat. Surface marked by 
fine, rounded, bifurcating strize, which become strongly curved as they 
approach the cardinal margin, about five or six of them occupying the 
space of 2 mm. on the margin. ‘These radiating markings are crossed 
by much finer, concentric striew, which are only visible through a 
magnifying glass, and by a few inconspicuous lines of growth. 
The dimensions of a rather small specimen are: length, 28 mm.; 
width, 32 mm.; convexity, 2.5 mm. 
STROPHEODONTA INDENTA (Con.). 
Plate XXVI., Fig. 12. 
1838. Leptena indenta Con., 2d Ann. Rep. N. Y. Geol. Surv., p. 117. 
Description.—Shell small, wider than long, semi-circular or semi- 
elliptical in outline, the hinge-lne equaling the greatest width. 
Pedicle valve convex, umbo prominent, flattened toward the cardinal 
angles, most elevated near the middle. Surface marked by strong, 
subangular, radiating ribs and by finer, concentric markings, which 
are strong in the furrows between the ribs, but scarcely recognizable 
upon the ribs themselves, giving to the furrows a conspicuous, indented 
appearance. Brachial valve not seen. 
The dimensions of the best-preserved specimen observed are: length, 
6 mm.; width, 7.5 mm. 
