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 varicoaa, pi. xlvi, figs. 12-23. 



Nuculites, Conrad (Geol. Surv. N. Y., Ann. Rep., p. 49. 1841). 

 [Types, Nuculites oblongatus, Conrad, and Nuculites cuneiformis, Connid.] 



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 striae 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 



