PANAMIC-PACIFIC PELECYPODA 201 



Genus DIPLODONTA Bronn, 1831 



Type species by subsequent designation, Gray, 1847, or Hermannsen, 

 1847, Venus lupina Brocchi. Miocene and Pliocene of northern Italy. 



Shell generally small, rounded, suborbicular to subtrigonal, with 

 median or slightly anterior umbones and prosogyrate beaks. Surface plain 

 or marked with concentric incrementals, smooth or minutely punctate. 

 Hinge as described for the family consists of two cardinal teeth in each valve 

 and of which the left anterior and right posterior are bifid or sulcate, no 

 laterals. Ligament external but sometimes becoming deeply immersed into 

 the hinge plate, often encroaching against the posterior cardinal teeth, the 

 nymphs raised into a platelike structure. No defined lunule or escutcheon. 

 Margins entire, the pallial line simple. Adductor scars distinct and sub- 

 equal in size. Shell white, overlain by a thin, usually brownish periostracum. 



In recent years, the name "Taras Risso, 1826" has been used to replace 

 that of Diplodonta but as Chavan, 1952 has indicated, Risso's name is 

 extremely questionable and should be suppressed. 



Two subgenera may be used for the Panamic species. 

 I. Shell rounded, subglose, plump, inequilateral. 



Subgenus Diplodonta, 



S.S. 



IT. Shell with subtrigonal umbones, the valves often depressed or 

 weakly convex, inequilateral. 



Subgenus Felaniella 



Diplodonta (Diplodonta) snprema, new species Plate 32, figures 2-2b 



Shell large (length 37 mm.), subcircular, convex, relatively thin, white 

 or cream-colored. Surface nearly smooth, marked only with fine, concentric 

 lines of growth and resting marks, and in some specimens very minute 

 radial lines and wrinkles may be observed. The hinge line is straight, long, 

 with the small beaks, subcentral, projecting slightly above it and placed 

 a little in front of the middle. The anterior side is a little shorter than the 

 posterior, narrower and broadly subtruncate at the end while the posterior 

 and ventral sides form part of a wide circular curve. Externally the valves 

 appear rounded and plump with the fullest inflation in a broad zone running 

 from the beak to the posterior-ventral margin. Hinge normal with the left 

 anterior and the right posterior cardinal teeth bifid. Cavity of shell deep, 

 the adductor scars and pallial line plainly marked. Adductor scars are of 

 nearly equal size, the anterior one somewhat more elongate, each placed 

 just below the end of the hinge. Pallial line entire, attached to the lower 

 end of each adductor. No lunule or escutcheon. 



Length 37.3 mm., height 31.2 mm., diameter 28.2 mm. 



Unusual amongst Recent species of the genus because of its large size 

 recalling several fossil forms usually referred to the subgenus Sphaerella. 

 It is most similar to T. subvexa (Conrad) from the Miocene of Maryland 

 which attains nearly the same size but has larger umbones and a more 

 circular form. 



Range — Panama. Panama Canal Zone: Palo Seco. 



