SYNOPSIS OF GENERA. 
XXXVll 
Valves moderately convex. Cardinal line about half the length of the valve, 
arching at the beaks. Umbonal slope often defined. 
Surface marked by more or less slender radii with narrow interspaces and 
by fine concentric striae of growth. 
Hinge narrow and long, furnished with a series of minute crenulations. 
Ligament apparently contained in a narrow groove along the cardinal border. 
Muscular impressions and pallial line unknown. 
This genus is distinguished by the transverse form of the species, the surface 
characters and the crenulations of the hinge, although this feature has been 
seen but in a single species, probably on account of the specimens having been 
preserved in sandstones and arenaceous shales. 
Examples: Pararca vmusta, pi. xciv, fig. 22. 
Pararca Sa.o, pi. lxx, fig. 17. 
Pararca erecta, pi. lxx, fig. 16 ; pi. xciv, figs. 20, 21. 
Cardiopsis, Meek and Worthen (Proc. Acad. Nat. Sci. Phila., p. 144. 1861). 
[Type, Cardiomorpha radiata, Meek and Worthen.] 
Eight valve gibbous, inequilateral, obliquely ovate or sub-circular. Beak 
prominent, incurved. Cardinal line short. 
Surface marked by radiating striae or costae, and concentric striae of growth. 
Hinge with one or two cardinal teeth. Ligament and muscular impressions 
unknown. 
Although in the original description of this genus the valves are said to be 
equal, it is probable that the description was written from observations based 
upon right valves alone. 
A comparison of specimens of this valve with the corresponding valves in 
the type-specimens of the genus Dexiobia , sIioavs a great similarity in form and 
surface characters, and it is probable that the two may prove to be generically 
identical. Also the genus Dualina (Barrande, 1881) seems to be congeneric 
with Dexiobia. 
Example: Cardiopsis radiata, pi. lxx, fig. 25. 
