PANAMIC-PACIFIC PELECYPODA 199 



coarse concentrics. Interior white or variously shaded with violet, heaviest 

 over the adductor scars and in a band outside the pallial line. Pallial sinus 

 small but distinct, pressed close against the posterior adductor scar. 



Length 39.5 mm., height 36 mm., diameter 28 mm. Holotype, British 

 Museum (Natural History). 



Recognized by its strongly convex and well-rounded valves, its poorly 

 defined umbonal angle and rather coarse but even concentrics. Rayed 

 markings often show near the ventral margin on removal of the periostracum. 

 As yet a little understood species. 



Range — Nicaragua to Ecuador. Panama: Bucaro; Guanico. Ecuador: 

 Cojimenes. 



]S^eoc.u-euji sp. Plate 28, figures 7, 7a 



Shell of medium size, short, ovate, cordate, the umbones prominent 

 and with the beaks placed near the anterior one-third. The posterior-dorsal 

 area is not well defined, there is only a low angle and a small change in 

 sculpture separating it from the main disk. Principal surface of the shell is 

 sculptured with coarse, rounded, concentric sulci, narrower than their in- 

 terspaces; these concentrics are regularly formed and distributed over the 

 general surface but become simpler and reduced in number at the posterior- 

 dorsal area, two sulci often joining to form a single one. Periostracum is 

 colored a light, greenish yellow with some darker concentric bands, and 

 below this faint purple rays are indicated. Hinge stained with purple and 

 in the deep interior. 



Length 34.6 mm., height 30.4 mm., diameter 2L3 mm. 



The above description is based on a shell in the Academy of Natural 

 Sciences Philadelphia, Collection (ANSP 155424) from the mouth of the 

 Guarara River, Panama, collected by Dr. J. Zetek. This form seems nearest 

 to A^ radiata (Hanley) but differs by its more rounded form. 



Superfamily LUCmACEA 

 Family DIPLODONTIDAE 



Shells of this family are generally small (length up to about 40 mm.), 

 suborbicular to subtrigonal, convex or depressed, thin, white. Surface smooth 

 or sculptured with fine growth lines, often minutely punctate or coarsely 

 granulose. Ligament external but sometimes becoming partly immersed 

 below the margin and attached to a flattened nymphal plate. Hinge 

 with two cardinal teeth in each valve, of which the left anterior and the 

 right posterior tooth are typically double or bifid. Adductor scars subequal 

 in size and connected by an entire pallial line which is often wide and 

 ribbon-like. 



From the Lucinidae, the members of this family differ by important 

 anatomical characters. On shell features, the most distinguishable characters 

 are the prominently double cardinal teeth (to which the group ov.es its 

 name, Diplodonta) and in its subequal adductor scars. Certain species 

 may be confused with the young of Dosima and Cyclinella but the absence 

 of a pallial sinus will distinguish them. 



Two genera occur in the southern Panama-Pacific faunal area. 

 L Shell suborbicular to subtrigonal, convex or depressed. Surface smooth- 



ish, sometimes submicroscopically punctate but not nodose. Ligament 



external, the nymphal area not deeply immersed. 



Genus Diplodonta 



