SYNOPSIS OF GENERA. 
XXYll 
crenulations. Just anterior to the posterior adductor are one or two small 
retractor impressions. The cavity of the umbo also usually shows three or 
four impressions of umbonal muscles. Pallial line simple. 
This genus differs very distinctly from Nucula in the anterior clavicle and 
absence of cartilage-pit. 
Examples: Nuculites oblongatus, pi. xlvii, figs. 1-12. 
Nuculites triqueter, pi. xlvii, figs. 17-28. 
Leda, Schumacher (Essai, p. 172, pi. xix, fig. 4. 1817). 
[Type, Area rostrata, Chemn.] 
Shell equivalve, inequilateral, transverse, often lunate, produced behind and 
rostrate. Anterior end rounded. Beaks often turned toward the posterior. 
Post-umbonal slope defined, extending from the beak to the posterior extremity, 
and often forming a nasute extension. 
Surface marked by more or less regular concentric striae. 
Hinge furnished with a line of minute teeth which is interrupted at the beak 
by the triangular cartilage-pit. Muscular impressions two, small. Pallial line 
simple or with a very slight sinus. 
Examples: Leda brevirostris, pi. xlvii, figs. 38-41. 
Leda rostellata, pi. xlvii, figs. 42-47. 
Leda pandoriformis, pi. xlvii, figs. 49, 58. 
Palasoneilo, Hall (Prelim. Notice Lamellibranchiata, 2, p. 6. 1870). 
[Type, Nuculites constricta, Conrad.] 
Nuculiform shells, transversely ovate or sub-elliptical, the posterior end 
extended, often sub-rostrate, with a more or less defined sulcus along the 
umbonal slope. Cardinal line arcuate. 
Surface marked by striae of growth, which are often latnellose and elevated 
into concentric ribs. 
Hinge furnished with a row of regular small transverse teeth, which is some¬ 
what interrupted beneath the. beak by a change in the direction of the teeth, 
