PANAMIC-PACIFIC PELECYPODA 427 



a. Ventral margin of the valves smooth. 



1. Surface sculptured with small, cord, or threadlike concentrics, their 

 interspaces with or without radial striae or pustules. 



Genus Caryocorbtda 



2. Surface more strongly sculptured with coarse concentric waves or 

 riblets. 



Subgenus Hexacorbula 



b. Ventral margin of valves serrated or radially striated. 



3. Shell generally coarse and heavy, its shape like that of Caryocorbvla. 

 Brackish. 



Genus Serracorbula 

 B. Shell oblong, rectangular or squarish, the posterior side short, flattened 

 as if truncated, 



c. Rostrum is well defined, bordered by a sharp angle or keel. Marine. 



4. Shell extremely thin and fragile. The rostral angle is a high sharp keel. 



Genus T enmcorbida 



5. Shell solid, the rostral keel a sharp angle. 



Genus Juliacorbula 



d. Rostrum is poorly defined or absent. Brackish. 



6. Shell often large, ovate, or cylindrical in shape, the two valves nearly 

 equal and similarly sculptured with growth lines only. Traces of lateral 

 teeth. 



Genus Panamicorbula 

 II. Valves unlike in size, shape, and sculpture. 



7. Valves rounded and unequal, the right valve is much larger, convex, 

 and with strong, concentric sculpture, the left smaller, flattened or 

 depressed, smooth or with some radial markings. 



Genus Varicorbula 

 Genus CARTOCORBULA Gardner. 1926 



Type species by original designation, Corbula alabamiensis Lea. Eocene 



of Claiborne, Alabama. 



Marine Corbulas with nearly equal, elongate-ovate to subelHptical 

 valves and small, adjacent, submedian, prosogyrate beaks, the posterior 

 side of the valves somewhat narrowed, rostrated, and often ending in a 

 twisted snout which may be formed by accessory shelly growth. The valves 

 are usually, slightly unequal in size, the right valve is a little larger, over- 

 lapping or clasping the left valve by its ventral margin, otherwise, the two 

 valves are much alike in shape and sculpture. The ligament is completely 

 internal; in the left valve, the ligament is attached to a projecting arm 

 or chondrophore set obliquely to the hinge margin and bearing two scars 

 divided by a central ridge or line. In the right valve, the ligament is attached 

 to scars on the inner wall and margin of the resilifer pit. Hinge teeth are 

 reduced to a single large cardinal tooth in the right valve fitting into a 

 socket along the resilifer pit in the left valve; there are no lateral teeth. 

 Pallial line mostly entire or its posterior section may show an obscure sinus. 



To this genus belong most marine Corbulas, recognized by their nearly 

 similar valves as to sculpture and convexity, and their pointed or rostrated 

 posterior end. The species are usually small and somewhat variable in shape, 

 hence often difficult to identify, and there is stil much uncertainty as to 

 the status of many named forms. Fresh specimens often have a strongly 

 twisted siphonal end or snout which is formed by the growing edge of the 

 periostracal layer, usually lost on a beachworn specimen. 



