202 AXEL A. OLSSON 



niplodonta (Diplodonta) discrepans Carpenter Plate 32, figures 7, 7a 



(b) Diplodonta fsemiaspera, var. discrepans Carpenter, 1857, Cat. Mazatlan Shells, 

 Brit. Mus., p. 102 Mazatlan. 



Shell small or of medium size, obliquely rounded, plump and relatively 

 thin. The umbones are full with the small beak placed a little anterior of 

 the middle, the anterior margin descending towards a widely rounded end. 

 The posterior side is wide and evenly rounded into the curve of the central 

 margin. External surface smooth or somewhat concentrically undulated by 

 growth incrementals and when unworn punctated with fine dots over the 

 whole disk including the dorsal submargins, where it is a trifle coarser. 

 Anterior-dorsal margins grooved in each valve. 



Length 16.6 mm.; height 14.7 mm.; diameter of a right valve 5 mm. 

 Isla del Gallo, Colombia. Most valves are smaller. 



This species has the microscopic punctation of D. soror C. B. Adams 

 of the western Atlantic, but the shell is smaller, more circular and the 

 anterior side is less expanded. Species with this type of surface punctation 

 have often been referred to Phlyctiderma, but the weaker hinge and wholly 

 external ligament are those of Diplodonta. 



A series of small Diplodonta was extracted by Carpenter from a 

 burrow in a Chama shell taken at Mazatlan, young and old specimens 

 tightly wedged together. Carpenter noted that the shells varied much in 

 shape, convexity, and in the position of the ligament; in some, the liga- 

 ment was conspicuously external, in others entirely hidden or internal. 

 Carpenter's specimens were not figured but manuscript drawings of the two 

 forms are in the molluscan library of the U.S. National Museum; to the 

 shell with the wholly internal ligament, Carpenter tentatively proposed 

 the name D. discrepans. Carpenter's manuscript figure, herein reproduced, 

 agrees well with specimens from Isla del Gallo, in southwestern Colombia. 



Range — Mexico to southwestern Colombia. Colombia: Isla del Gallo. 



Subgenus FELANIELLA Dall, 1899 



Type species by original designation, Felania usta Gould. 



Shell heavier, compressed, smooth externally or with relatively coarse, 

 concentric sculpture. Surface usually covered with a dark periostracum. 

 Valves are less equilateral. 



Diplodonta (Felnniella) obliqua Phllippi 



Diplodonta obliqua Philippi, 1846, Zeitschr. f. Malakozool., p. 20 (Mazatlan). 

 Lucina calculus Reeve, 1850, Conch. Icon., vol. 6, Lucina, pi. 11, fig. 68 (Gulf of Nicoiya). 

 Diplodonta {Felaniella) obliqua Philippi, Dall, 1901, Proc. U.S. Nat. Mus., vol. 23, No. 



1237, p. 796. 

 Taras {Felaniella) obliquus (Philippi), Hertlein and Strong, 1947, Zoologica, vol. 31, 



pt. 4, No. 10, p. 131. 



Generally small (max. about 11 mm.), white, obliquely subovate, in- 

 equilateral, the anterior side highest, rounded, the posterior shorter, 

 obliquely subtruncate, the ventral margin well rounded. Valves depressed 

 to moderately convex, mainly in the umbonal region, the dorsal slopes, 

 somewhat depressed. Sculpture formed by irregular, fine and coarse con- 

 centrics over the entire disk. 



