PANAMIC-PACIFIC PELECYPODA 119 



Key to subgenera Scolimytilus 



I. Umbonal slope convex or rounded, the surface ribbing rather fine and 

 fairly uniform. Posterior adductor and pallial line impression, fused and 

 lobate in shape. 



Subgenus Scolimytilus s.s. 



II. Umbonal slope often strongly arched, the ribbed sculpture divided, 

 coarse on the dorsal side. Adductor and pallial impression narrow and 

 ribbon-like. 



Subgenus AeidimytUus 



Scolimytilns (Scolimytilus) playaseusis (Pllsbry and Olsson) 



Plate 13, figures 2-2c 



Modiolus (Brachydontes) playasensis Pilsbry and Olsson, 1935, Nautilus, vol. 49, No. 

 1, p. 17, pi. 1, fig. 4 Playas, Ecuador. 



Shell with an average length of about 20 mm., medium-elongate, 

 obliquely wedge-shaped, with nearly terminal beaks and a straight dor- 

 sal margin, forming with the posterior margin a corner angle of about 

 120 degrees. The ventral side is flattened or impressed, rounding above into 

 a fairly pronounced umbonal slope, highest in the middle zone and from 

 there sloping downward to form a rounded posterior end. Ligamental scar 

 linear, narrow, straight, as long as the dorsal margin, the small beak at its 

 anterior end, with one or more medium-sized teeth below it and a few 

 small crenulations on the anterior margin which project a little beyond it. 

 Sculpture is produced by low, smoothish, radial riblets of nearly uniform 

 strength over the whole surface except along the middle part of the um- 

 bonal slope which may be smooth. Inner margins of valves crenulate con- 

 forming to the outside ribbed sculpture. Periostracum of a rich brown color, 

 glossy and where peeled off, the color underneath is a purple-brown. In- 

 terior of shell of a pale brown, the pallial and muscle impressions distinct, 

 and generally as figured. 



The type measures: length 20.7 mm., height 9 mm., diameter 8.7 mm., 

 ANSP 164617. 



This is a local species, probably restricted to the inner shores 

 of the Gulf of Guayaquil. From other species of the genus, it will 

 be separated by the rich brown color of its glossy periostracum, its lower 

 riblets, nearly uniformly developed over the whole disk, and by its wider, 

 wedge shape, its ventral margin straight or but little impressed. Pallial 

 and adductor impressions are generally well marked. 



Range — Shores of the Gulf of Guayaquil, Ecuador and Peru. Ecuador: 

 Playas; Posorja. 



Scolimytllns (Scollmytllns) esmeraldensis, new species Plate 13. figures 1-lb 



Shell of medium size (length from 14 to 17 mm.), elongate, mytiliform, 

 the beaks anterior and placed nearly at the end, the anterior end itself 

 being short, narrowed, and pinched. The dorsal or ligamental margin is 

 long, usually a little bowed in the middle, the scar of the ligament below 

 it white, enameled and extending nearly its whole length. Beyond the dor- 

 sal or ligamental margin, the posterior side descends obliquely and is broad, 

 appearing as if narrowly rounded or subtruncated at the end. The ventral 

 side is usually flattened or impressed in the middle, sometimes strongly 



