XXVI 
SYNOPSIS OF GENERA. 
Nucula, Lamarck (Hist. An. Sans. Vert. 1799). 
Shell small, equivalve, inequilateral, trigonal or transversely elliptical or 
sub-circular. Anterior or posterior extremity sometimes produced, usually 
rounded. Beaks anterior or posterior to the middle of the length, often sub¬ 
central. Cardinal line arcuate. Escutcheon marked. 
Surface marked by concentric striae, which, in some species, are regular 
and rugose. 
Hinge furnished with a triangular, spoon-shaped cartilage-pit beneath the 
beaks, with a series of small transverse teeth on each side. There are two 
principal muscular impressions in each valve, with usually a smaller retractor 
scar adjacent, and also the cavity of the beaks often shows several pits for 
the attachment of umbonal muscles. Pallial line simple. 
Most of the species described in this volume have the beaks anterior to the 
middle. One species, N. lamellata, shows the beaks posterior to the centre, but 
in a few specimens of this species it is sub-central. 
Examples: Nucula Randalli, pi. xlv, figs. 6-10,16, 23, 26, 27; pi. xciii, figs. 1-3. 
Nucula varicosa, pi. xlvi, figs. 12-23. 
Nuculites, Conrad (Geol. Surv. N. Y., Ann. Hep., p. 49. 1841). 
[Types, Nuculites oblongatus, Conrad, and Nuculites cuneiformis, Conrad.] 
Shell equivalve, inequilateral, transverse. Anterior end rounded. Posterior 
end sometimes obliquely truncate and pointed. Beaks anterior. Cardinal line 
arcuate. Post-umbonal slope rounded or angular. 
Surface marked only by concentric strise in all the known species. 
Hinge furnished with a row of transverse narrow teeth, beginning at the 
anterior muscular scar and extending without interruption posteriorly as far 
as the posterior scars. Ligament external, contained in a narrow groove along 
the margin of the hinge. Anterior muscular scar deeply impressed, separated 
from the cavity of the shell by a vertical or slightly oblique clavicle, or parti¬ 
tion, extending about two-thirds the distance from the beak toward the base. 
Posterior scar elongate, situated just below the termination of the hinge 
