Nevi Lmnellibranchia ta . — U I rich . 9 7 
forms, whose distinctness is not 3'et fully established, will be 
postponed till the appearance of the final volume. 
Cleionychia, n. gen. 
{Clf'io, I close, onyx, a claw.) 
Shell equivalve, subalate posteriorly ; beaks terminal, more or 
less prominent, slightly incurved. Cardinal line straight, rather 
long, usually forming an angle of less than 90° with the anterior 
side. Ventral and posterior margins more or less rounded. 
Surface marked concentrically only, and sloping very gentl}' to 
the posterior and hinge margins, and proportionately very rapidly 
on the anterior side. The latter, especially in casts of the in- 
terior, flattened or even concave. No byssal opening, the mar- 
gins closing tightly all around. Muscular impression situated in 
the postero-cardinal third, large, bilobed, the lower lobe much 
larger than the upper. Pallial line entire, extending from the 
muscular scar to the cavit}' of the beak. Hinge plate edentulous, 
excavated longitudinally for a linear ligament, with the inner 
edge thickened and bifurcate at the posterior extremity'. A small 
pit just beneath the beak. Upper part of anterior edge of shell 
thickened. 
Type: Amhonycliia JanieUosa Hall. 
This genus will include beside the type and the species next 
described, Amhonycliia erecta and A. attcnnata Hall, from the 
Trenton of Wisconsin, ^1. mytiliokhx Hall, from the Chazy of 
New York, Fterinca undata Emmons, and Amhonycliia aniyg- 
dalina Hall, from the Trenton of that state. Further, it is highl}' 
probable that A. nit Ida and superha, described by Billings from 
Anticosti, and one or two concentrically marked Upper Silurian 
species referred by Lindstrom to Amhonycliia , also should be 
placed under Cleionychia. 
As now understood the new genus is distinguished from ^1;»- 
honychia (1) by the absence of radiating plications or striae, (2) 
the absence of a byssal opening in the anterior end, (3) the less 
central position of the muscular scars, and (4) liy the absence of 
distinct hinge teeth. 
Cleionychia RiioMnoiDEA. n. sp. 
Plate VII, Figs. 1 aud 2. 
Shell, as seen in casts of the interior, of medium size, very 
oblique, rhomboidal in outline, the anterior and p^jstenor and the 
