SYNOPSIS OF GENERA. xxxvil 



Valves moderately convex. Cardinal line about half the length of the valve, 

 arching at the beaks. Urabonal 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 

 chtaracters 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 Sao, pi. Ixx, fig. 17. 

 Pararca ereda, pi. Ixx, 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.] 



Right 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, shows a great similarity in form and 

 surface characters, and it is probable that the two may prove to" be generically 

 identical. Also the genus Dualim (Barrande, 1881) seems to be congeneric 

 with Dexiobia. 



Example : Cardiopsis radiata, pi. Ixx, fig. 25. 



