xxxviii SYNOPSIS OF GENERA. 



Lunulicardium, Minister (Beithige zur Petrefakten-Kunde, 3d heft, p. 69. 



1840). 



Shell equivalve, inequilateral, longitudinally or transversely suh-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 curtum, pi. Ixxi, figs. 18-23. 



Lunulicardium iransversum, pi. xciv, figs. 24, 25. 

 Lunulicardium fragile, pi. Ixxi, 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. 



