Io6 BULLETIN OF THE LABORATORIES 
present a very different appearance, so as often to suggest distinct spe- 
cies, as may be noticed by the following descriptions. 
Ventral valve, length 6.7 mm, width 8 mm. Valve moderately 
convex, the cardinal extremities compressed, umbo moderately promi- 
nent. The corresponding dorsal valve is moderately concave, greatest 
concavity near the beak. 
Ventral valve, length ii mm, width 14 mm. Valve strongly con- 
vex, especially at t'he umbo which is about one-third the distance from 
the beak to the anterior margin ; the cardinal extremities compressed. 
Ventral valve, length 15 mm, width 19 mm. Valve convex es- 
pecially at the umbo, which is one-fourth the distance from the beak, 
the anterior, lateral, and postero-lateral extremities flattened or com- 
pressed. The corresponding dorsal valve is slightly concave, its great- 
est concavity being about one-fourth the distance from the beak. The 
cardinal extremities are quite flat. 
Ventral valve, length 18 mm, width 24 mm. Valve with cardinal 
extremities quite flat ; the regions about the umbo, which is one fifth 
the distance from the beak, convex; moderately concave beyond the 
centre, towards the anterior and lateral margins. These margins in 
the dorsal valve are slightly convex. 
.Ventral valve, length 25 mm, width 33 mm. Concave in general 
with a convex umbo, whose greatest convexity lies at about one eighth 
the distance from the beak. The dorsal valve concave with the ante- 
rior and lateral regions moderately convex. This is of the form of the 
New York specimens which formed the type of this species. These 
never seem to arrive at the stage presently to be described; the stria- 
tions are also coarser than those of the Ohio forms. 
Ventral valves, length 28, width 35. Valves convex for about 
half their length, the umbo being very close to the beak, beyond the 
middle anteriorly and laterally they become strongly recurved, or 
geniculate in some individuals. The corresponding dorsal valve is 
moderately concave for half its distance, then strongly bent or genicu- 
late for the remainder of the distance' towards the anterior and lateral 
margins. 
Shell marked by numerous, fine, radiating strise; in young speci- 
mens certain of the striae at more or less regular intervals become 
more prominent, in the typically strophomenoid manner. In older 
specimens, the prominence of these striae no longer is sufficient to at- 
tract attention, or is even entirely lost. After the removal of the thin 
