278 AXEL A. OLSSON 



tending to the posterior end; ends and ventral margin rounded; exterior 

 surface ornamented by fine lines of growth, strongest near the margin, 

 very faint near the beaks, crossed by numerous, almost microscopic, radial 

 grooves which divaricate along radial lines extending from the beaks to the 

 posterior and anterior ends of the basal margin, obsolete near the beaks; 

 hinge and ligament strong, normal for the genus and subgenus; pallial 

 sinus short, fairly wide, rounded at the end, ascending, projecting forward 

 to about one-third the length of the shell; interior white; margin smooth. 



Length 53.8 mm., height 46 mm., convexity (double valves) 34.2 mm. 

 (Hertlein and Strong, 1948.) 



This species is somewhat similar to P. pollicaris but is higher in pro- 

 portion to its length, more trigonal in shape, and more convex. It also 

 resembles P. carbaseus Guppy from the Miocene of Jamaica. It is evidently 

 a rare species. 



Range — Gulf of California, south to Costa Rica. 



Genus AGRIOPOStA Dall, 1902 

 Type species by original designation, Cytherea texasiana Dall. Plate 

 40, figure 1. Gulf of Mexico. 



Shell cordate, convex, thin, inequilateral, with large, full umbones, 

 the beaks placed anteriorly and noticeably recurved. The lunule is large, 

 heart-shaped, defined by a faint line. Color is usually a gray white, smooth 

 or chalky, and marked with fine, concentric grovvth threads. The hinge 

 pattern is like that of Pilar and Lamelliconcha with three cardinal teeth 

 in each valve and a small, pimple-Hke, anterior lateral tooth in the left 

 valve, widely removed from the nearest cardinal tooth by an open or empty 

 pit. In the right valve, the upper ends of the anterior and posterior cardinal 

 teeth are joined together and form a single, large, hook-shaped lamina 

 and which extends over the central cardinal tooth like a roof, the tooth 

 standing wholly free below it. In the left valve, the anterior and central 

 cardinal teeth are joined and form an unequal sided, inverted V, the central 

 arm of which is the larger and heavier: above, the apex of this V-shaped 

 tooth is separated from the hinge margin by a deep, narrow sulcus. The 

 pallial sinus is deep, its end sharp. 



This genus differs from Pitar in that the upper ends of the cardinal 

 teeth do not touch the margin above but stand free and separated from 

 the margin by a deep, narrow slit or sulcus; also by the much smaller, 

 anterior lateral tooth bordered behind by a false socket. 



Subgenus PITARELLA Palmer, 1927 

 Type species by original designation, Pitaria gatunensis (Dall). Mio- 

 cene of Panama and Panama Canal Zone. 



Like Agriopoma, s.s., the shell is cordate in shape, convex, thin, its 

 surface smooth or marked with crowded, concentric threads. Lunule is 

 heart-shaped, bordered by a fine line. Hinge similar to that of Agriopoma, 

 S.S., but with the anterior side of the hinge plate shorter as if cut off or 

 excavated by the deep, anterior adductor scar, hence, the anterior scar is 

 crowded against the cardinal teeth, its dorsal side forming a curved trans- 

 verse lamella joined at its end to the base of the cardinal tooth forming a 

 deep, empty space or pit above it. 



