48 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



base, more prominent than those of left valve. Radial elements most 

 prominent at valve margin, becoming coarser with age. 



Cardinal area. — Composed of central resilifer bounded laterally by 

 flat to slightly arched, striated, triangular lateral cardinal plates. 

 Resilifer of left valve shallow (most common) to moderately concave, 

 triangular to subtriangular with bent dorsal apex, slightly flared base ; 

 moderately narrow (common) to broad (rare). Lateral cardinal 

 plates subequal to moderately unequal (on shells with prominent 

 posterior auricle), posterior plate largest; each plate larger than 

 resilifer in most valves. Narrow marginal cardinal troughs commonly 

 developed between lateral cardinal plates and margin of valve, most 

 common on left valves. Cardinal area of right valve similar to left 

 except for resilifer, which is commonly shallow (concave), flat, or 

 rarely raised into a low midcardinal fold. Majority of right valves 

 with shallow resilifer bounded ventrally by a raised lip (partial mid- 

 cardinal fold). 



Cardinal area marked with moderately strong, crowded, irregularly 

 spaced, horizontal striae, raised lines, and narrow ridges of various 

 sizes. Horizontal elements transected by faint, crowded, vertical lines. 

 Ornament most strongly developed in adult shells, on lateral cardinal 

 plates. Cardinal area color-banded on some specimens, with narrow, 

 dark horizontal lines on lighter background. Hinge line short to 

 moderately long (in presence of well-developed posterior auricle), 

 straight to slightly curved (common), concave toward center of valve 

 except below resilifer, where slightly convex (pi. 5, fig. 13). 



Denticles. — Present on inner dorsolateral margins of both valves ; 

 small, simple, subround to elongate perpendicular to margin, in some 

 valves their trace visible on lateral margin of shell, crossing lamellae. 

 Interspaces equal to or slightly wider than denticles. Denticles 

 situated on commissure and in narrow trough just inside it, generally 

 restricted to dorsolateral margins, rarely extending well down margin, 

 generally faint on young shells, best developed on adults, becoming 

 indistinct on large old valves. 



Commissure, interior surface. — Commissure situated at margin of 

 left valve, just inside margin of right valve ; normally undulating, 

 rarely zigzag or flattened (old specimens). Plicae faintly expressed on 

 interior of left valves near margins, more prominent on interiors of 

 right valve, extending to center as rounded folds, sulci. Scattered, 

 sinuous, pallial grooves faintly developed on a few valves. 



Muscle scar. — Monomyarian, insertion area of moderate size, well 

 defined, subcrescentic to comma-shaped if not worn (pi. 5, fig. 13), 



