xxxvi SYNOPSIS OF GENERA. 



Praecardium, Burrande (Syst. Sil. de laBoheme, pt. 1, vol. vi, p. 141. 1881). 



Shell equivalve, more or less inequilateral, longitudinally or transversely 

 elliptical or trigonal. Beaks prominent and incurved. 



Surface marked by concentric striae of growth, and by strong elevated 

 radii, separated by distinct interspaces. 



Posterior to the beaks is a small area which carries a series of vertical 

 nearly parallel teeth. Other characters of the interior unknown. 



The species of this genus differ from Panenka in the characters of the hinge, 

 and from Pararca in form, surface markings, and in the cardinal area posterior 

 to the beaks. 



Example : Pracardium vetustum, pi. Ixx, figs. 18-20. 



The genus is represented in America, so far as known, by the single species 

 cited above, occurring in the shales of the Portage group. 



Paracardium, Barrande (Syst. Sil. de la Boheme, pt. 1, vol. vi, p. 137. 



1881). 



Shell equivalve, more or less inequilateral, sub-circular or broadly elliptical. 

 Posterior side sub-truncate, resembling in this respect some broad forms of 

 Lunulicardium. Valves moderately convex. 



Surface marked by numerous fine radii with narrow interspaces, and by tine 

 striae of growth. 



Under the beaks is a small cardinal area, the margin of which is minutely 

 crenulated. Muscular scars and pallial line unknown. 



The shells of this genus differ from those of the genus Pararca in the pres- 

 ence of a cardinal area. Pararca has a longer hinge-line and more extended 

 line of crenulations, and the form is more transverse. 



Example: Paracardium Doris, pi. Ixx, figs. 10, 11. 



Pararca, n. g. [Types, Pararca venusta. Hall, and Pararca erecta, iBEall.] 



Shell equivalve, inequilateral, transversely sub-elliptical or rhomboidal in 

 outline. Posterior end often sub-truncate. Anterior end short and rounded. 



