356 PALEOZOIC PALEONTOLOGY. 
the cardinal extremities, with a narrow mesial groove or sinus on the 
summit of the mesial elevation: cardinal area narrow and linear; the 
beak rather blunt, incurved. Brachial valve flattened on the umbo 
and toward the lateral margins, depressed medially in a broad sinus, 
along the median line of which is a small, elevated ridge. The sur- 
face of perfectly-preserved specimens is marked by fine, concentric 
and radiating strive. 
The dimensions of an imperfect pedicle valve are, approximately: 
length, 8 mm.; width, 10 mm.; convexity, 3.5 mm. 
METAPLASIA PLICATA MN. Sp. 
Plate XLVIII., Figs. 7-12. 
Description.—Shell subcircular or subelliptical in outline, wider 
than long, greatest width along the hinge-line, cardinal extremities 
angular. Pedicle valve strongly convex, having a prominent mesial 
elevation extending from the beak to the front margin, with a 
rounded longitudinal depression. From this mesial elevation the 
surface slopes away somewhat gradually to the lateral margins and 
curves rather abruptly to the cardinal margin; the beak is prominent 
and strongly incurved; the cardinal area is of moderate width, 
sharply defined and extending to the cardinal extremities. Brachial 
valve depressed-convex on the umbo, flattened toward the cardinal 
margins, flat or concave laterally, depressed medially toward the front 
in a broad sinus, with a rather broad, rounded plication along the 
median line. Surface of each valve marked by from eight to twelve 
rounded plications, of which the two median ones of the pedicle valve 
are much the largest and form the two sides of the mesial elevation 
of that valve. The three median depressions of the pedicle valve— 
that is, the one lying between the two larger median plications and 
are conspicuously broader than those 
the two adjacent lateral ones 
beyond; the plications on the lateral slopes gradually decrease in size 
to the cardinal margin. On the brachial valve there aré three broad 
median plications corresponding to the three broad furrows of the 
opposite valve, the one on the median line, and the adjacent ones 
forming the sides of the mesial sinus; beyond these the plications 
decrease gradually in size to the cardinal margin. Besides the pli- 
cations the surface is marked by indistinct concentric strive, by ex- 
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