xxxviii SYNOPSIS OF GENERA. 
Lunulicardium, Munster (Beitrage zur Petrefakten-Kunde, 3d heft, p. 69. 
1840). 
Shell equivalve, inequilateral, longitudinally or transversely sub-elliptical, 
sub-circular or trigonal. Posterior side obliquely truncate by the cardinal 
hiatus, with the margin often reflexed and produced. Beaks pointed, sub-cen¬ 
tral or anterior. Cardinal line marked by a lunate hiatus in each valve which 
was probably occupied by the ligament. 
Surface marked by concentric striae and usually by more or less well-devel¬ 
oped radii with distinct interspaces. Some species also show a minutely pustu- 
lose character of the surface. 
Ligament external, occupying the hiatus of the cardinal line. 
Hinge, muscular scar and pallial line unknown. 
Examples: Lunulicardium curium , pi. lxxi, figs. 18-23. 
Lunulicardium transversum , pi. xciv, figs. 24, 25. 
Lunulicardium fragile, pi. lxxi, figs. 1-14. 
Paracyclas, Hall (Geol. Surv. N. Y.; Report of the Fourth District, 
p. 171. 1843). [Type, P. elliptica.] 
Shell equivalve, sub-equilateral, sub-orbicular or broadly sub-elliptical. An¬ 
terior end regularly rounded ; posterior end rounded or sub-truncate, somewhat 
more produced below than the anterior; beaks small and low, generally rising 
little above the hinge-line. 
Hinge-line short. Post-cardinal slope more or less defined by an oblique 
furrow or depression, which sometimes leaves the extremity sub-alate. 
Surface concentrically striated; sometimes with strong concentric ridges 
marking the exterior. 
Structure of hinge not fully observed. Ligament supported on each side, 
internally, by a narrow plate, and leaving in the cast two diverging grooves, 
directed forward from the beak. Muscular impression on the post-umbonal 
slope. Pallial line parallel with and a little within the margin of the shell. 
