168 AMERICAN MARINE CONCHOLOGY. 
elongated within the pallial line, posterior oblong ; umbonal area with an 
oblique furrow. LuciIna. 
Shell globular, posterior side furrowed or angulated, umbones much re- 
curved ; lunule short or indistinct ; ligament to a certain extent external, 
placed in a groove on the hinge-line, and outside: the hinge-plate; teeth 
altogether wanting. CRYPTODON. 
Shell suborbicular, smooth; ligament double, rather long, submarginal ; 
hinge-teeth 2.2, of which the anterior in the left valve and posterior in 
the right, are bifid, lateral teeth none; muscular impressions polished, 
rounded. Mysra. 
Shell small, thin, suborbicular, closed; beaks small; margins smooth ; 
ligament internal, interrupting the margin, or on the thickened margins; 
cardinal teeth 1 or 2, laterals 1-1 in each valve. KELEIA,. 
Shell oblong, inequilateral,‘anterior side very short; ligament concealed 
between the valves; hinge-teeth 2-2. TURTONIA. 
Shell small, thin, oblong, anterior side longest; hinge line notched ; liga- 
ment internal, between two laminar diverging teeth. MOonTACUTA. 
Shell equivalve, orbicular, subequilateral, compressed, gaping slightly at 
the sides ; surface of valves shagreened or smooth; margins plain ; beaks 
acute. Hinge composed of a pair of teeth-like lamin on each side of a 
triangular, central excision in one valve; a primary apical tooth in front 
of a subtriangular excision of the hinge-margin, and flanked on each side 
by a sublateral lamina in the other. Pallial impressions simple. 
LEPTON. 
Genus LUCINA, Bruguiere. 
Encyc. Meth., t. 284. 1792. 
Animal with the mantle freely open below; siphonal orifices 
simple; mouth minute, lips thin; gills single on each side, very 
large and thick; foot long, cylindrical, pointed, slightly heeled at 
the base. 
1. L. FILosa, Stimpson. Fig. 426. 
Shells of New England, 17. 1851. 
Lucina radula, Gould (not Montagu), Invert. Mass., edit. i. 69. 1841. 
Lucina contracta, De Kay (not Say), Nat. Hist. New York, 214, t. 27, f 
275. 1848. 
Shell orbieular, depressed; surface with numerous remote con- 
centric laminated strize; lunule depressed lanceolate; lateral teeth 
obsolete. White or light brown. 
Length and height 1.5 inches. 
New England. 
