PANAMIC-PACIFIC PELECYPODA 261 



this species and several closely allied forms have a wide geographic distri- 

 bution extending from Chile northward to California and through the 

 Caribbean region to Trinidad. It is a common fossil in the Pleistocene 

 tablazos of Peru. 



The typical form of Dosinia ponderosa as it occurs in the Gulf of 

 CaHfornia has been well figured by Grant and Gale (plate 15, figs, la, lb, 

 Ic). It is a medium to large, coarse, heavy shell with an excentric circular 

 outline. Its lunule is so deeply impressed that its lower margin appears to 

 have been turned sharply inward, almost at right angles, while the anterior 

 margin below it is strongly produced, expanded and rounded. The hinge 

 plate behind the cardinal teeth is high and flat, the ligamental scar so deeply 

 inset that the shell margin above it overhangs widely. If the valves are not 

 carefully separated the hinge plate is always broken or damaged. The 

 surface of the disk is usually smooth and polished over the middle zone; 

 the concentric sculpture of strong, ribbon-like concentrics shows best on 

 the sides. Shells from the southern part of the Panamic region have a more 

 perfect circular or quadrate outline, the lunule is less deeply sunken, the 

 anterior margin below the lunule shorter, while the surface sculpture is 

 stronger, the concentrics persisting across the middle of the disk. This 

 latter form occurs frequently as fossil in the Pliocene and upper Miocene 

 beds of northern Peru and in Ecuador, and was named Dosinia grandis by 

 Nelson. 



Range — Lower California to Peru and the Galapagos Islands. Panama: 

 Panama City; Guanico; Pearl Islands. Ecuador: Santa Elena; Manta; 

 Jaramijo; Esmeraldas. Peru: Zorritos. 



Daslnia (Doslnidla) dnnkeri (Phlllppi) Plate 42, figures 3-3b 



Cytherea dunkeri Philippi, 1844, Abbild. und Beschreib. Conchylien, bd. 1, Cytherea, p. 



170, pi. 2, fig. 5 Mexico. 

 Artemis simplex Hanley, 1845, Proc. Zool. Soc. London, p. 11 Panama. 

 Artemis dujikeri (Philippi), Reeve, 1850, Conch. Icon., vol. 6, Artemis, pi. 6, fig. 34. 

 Dosinia dunkeri (Philippi), Romer, 1862, Mon. Molluskengattung Dosinia, ScopoH, p. 



17, taf. Ill, figs. 3, 3a, 3b. 

 Dosinia (Dosinidia) dunkeri (Philippi), Hertlein and Strong, 1948, Zoologica, vol. 3, 



pt. 4, p. 165. 



Shell small or medium in size (length 40 to 55 mm.), subcircular to 

 subovate, solid, moderately convex, white. The beaks project strongly over 

 a deeply sunken lunule. Surface sculptured with regular, flattened, slightly 

 shelving concentric ribbons which are of uniform strength over most of 

 the disk. Adductor scars are relatively large, polished, the anterior one 

 more deeply impressed. Pallial sinus is relatively large, straight and reaches 

 a little beyond the middle of the shell cavity. 



Length 55 mm., height 53.1 mm., diameter 28 mm. Santa Elena, 

 Ecuador. 



The commonest species of Dosinia in the Panamic zone, easily re- 

 cognized by its circular form and regular sculpture. 



Range — Costa Rica: Puntarenas. Panama: Panama City; Pearl Islands. 



