LAMELLIBRANCHIA TA. 
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Six characteristic specimens have the following dimensions, respectively: 
Length 46, 37, 35, 37, 28 and 38 mm.; height 27, 22.5, 25, 28, 21 and 31 mm. 
This species differs from M. gregarius in its broader cardinal slope, less 
oblique posterior margin, larger size and stronger surface striae. M. complanatus 
is a larger and more elongate form, the posterior and anterior extremities more 
regularly curved, the striae irregular and fascicled, the beaks more elevated and 
the posterior margin more obliquely truncate. 
Formation and localities. In the shales of the Hamilton group throughout the 
range of this formation in the State of New York, but more abundant in the 
eastern and central portions; also from the same horizon, near Cumberland, 
Md.; and in the Chemung group, near Elmira and Ithaca, N. Y.; and from 
Susquehanna and Tioga counties, Pa. 
Microdon (Cypricardella) gregarius. 
PLATE LXXIII, FIGS. 1-6 : AND PLATE LXXIV, FIGS. 1-4. 
Microdon yregaria, Hall. Prelim. Notice Lamellibranchiata, 2, p. 32. 1870. 
Microdon ( Microdonella ) ( Eodon ) gregarius, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 
73, figs. 1-6. 1883. 
Shell small, rhomboid-ovate; length less than one-third greater than the 
height; basal margin regularly curving, sometimes nearly straight toward 
the post-basal angle; posterior margin obliquely sub-truncate, slightly curv¬ 
ing ; cardinal margin gently arcuate or nearly straight. . Anterior end 
declining abruptly from the beaks, with the extremity narrow and abruptly 
rounded. 
Valves depressed-convex, more convex at the umbo and on the umbonal 
ridge. 
Beaks situated at less than the anterior third of the length, small, low, 
closely appressed, scarcely rising above the hinge-line. Umbonal ridge dis¬ 
tinct, hut not strongly defined, continuing to the post-basal angle. 
Test thin, marked by fine, concentric striae, which are somewhat fasciculate 
on the anterior portion of the shell. 
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