272 AXEL A. OLSSON 



or with incised lines. The hinge has three cardinal teeth in each valve which 

 stand nearly free and are not joined to the margin above them; also a 

 larger, anterior lateral tooth in the left valve and its deep, containing socket 

 in the right. Lunule cordate, bordered by an incised line and also shown 

 by a change of sculpture and of color; there is no escutcheon. Pallial sinus 

 deep. 



Key to genera of the Pitarinae 



I. Shell solid, the surface with a smooth, porcellaneous luster or weakly 

 sculptured with low, rounded concentrics. 



A. Shell generally large, smooth, and glossy. Scar of the pedal retractor 

 muscle on the undersurface of the hinge plate and below the anterior 

 cardinal tooth is distinct and deep. 



1. Shell ovate to elliptical in shape, the surface color plain and simple 

 or with a strong, carpet-like pattern of radial rays and zigzag markings. 



Genus Macrocallista 



B. Shell smaller, the surface smooth, plain or with low concentrics. Pedal 

 retractor scar not deeply impressed. 



2. Valve margins smooth. 



Genus Tine tor a 



3. Inner margin of valves cut by small, tangential lines. 



Genus Transennella 

 II. Valves thin or of medium weight. Surface plain, marked only with 

 growth lines or more heavily sculptured with concentric lamellae or 

 ridges. 



C. Surface sculptured principally with fine, growth lines. 



4. Shell cordate, convex, with large, full umbones. 



Genus Pitar 



D. Surface sculptured with coarse concentric riblets, rounded or lamellar. 



5. Posterior-dorsal area is poorly defined, the concentric riblets not inter- 

 rupted by it. 



Genus LameUiconcha 



6. The posterior-dorsal area is set apart sharply by the umbonal angle 

 armed with large spines or stumplike scales. 



Genus Hysteroconcha 



Genus 3IAC0BOCALLISTA Meek, 1876 



Type species by monotypy, Venus gigantea Gmelin. { = V. nimbosa 

 Solander). Recent, Atlantic and the Gulf Coast of the United States. 



Shell elongate-ovate, subsolid, flattened to convex, smooth and polished 

 except for growth lines and in life covered by a thin, vernicose periostracum. 

 Lunule is narrowly cordate, defined by an incised line. No differentiated 

 escutcheon. Hinge of each valve has three cardinal teeth, the left posterior 

 slender, adjacent to the nymph and partly fused to it, the anterior right 

 cardinal tooth narrow and set vertical as if to close-off the lateral socket; 

 there is an anterior, lateral tooth in the left valve and its fitting socket in 

 the right. Anterior pedal scar seated on the hinge plate below the anterior 

 lateral tooth. The pallial sinus is moderate. Ventral margins smooth. 



Subgenus MEGAPITARIA Grant and Gale, 1931 

 Type species by original designation, Cytherea aurantiaca Sowerby. 



