460 AXEL A. OLSSON 



Cjathodonta tunbeziana, new species Plate 83, figures 1, la 



Shell ovate, the umbones high and prominent, subcentral, with the 

 beaks placed a trifle closer to the posterior side, the middle of the ventral 

 margin of the right valve with a pronounced bulge, the ventral margin of 

 the left valve more evenly curved. Posterior side short, depressed, bordered 

 by a low umbonal angle. Sculpture consists of small wavelike undulations, 

 concentric to the lines of growth except on the anterior-umbonal slope 

 where they are slightly oblique; they are best developed on the umbones 

 of the right valve but fade-out rapidly towards the ventral margin. The left 

 valve is less convex than the right, its sculpture weaker. The granulation 

 is principally developed on the posterior slope where it may be quite coarse 

 and without a fixed pattern of distribution; on the posterior umbonal angle 

 the granules often show a linear arrangement; elsewhere the submicroscopic 

 sculpture is reduced to minute, concentric threads. Pallia! sinus short, not 

 quite extending to the line of the beak. 



Length 37.8 mm., height 32.3 mm., semidiameter 8 mm. a right valve, 

 Tumbez, Peru. Holotype, ANSP 218952. 



This species differs from C. undulata peruviana by its shape, unequal 

 valves, and in sculpture. 



Range — Peru. Peru: Tumbez; Punta Picos; Mancora. 



Family PERIPLOMATIDAE 



This family includes subnacreous shells, often thin and fragile, white 

 and pearly within, rounded or oblong-ovate in shape, inequivalve, the ri<^ht 

 valve more strongly convex, the other often flattened and depressed. The 

 posterior side is usually short, lower, often flexuous, but not open or gaping 

 at the end. Hinge edentulous, the ligament wholly internal, attached to a 

 forward projecting spur or chondrophore in each valve, strengthened below 

 and behind by a rib or clavicle; closely connected with the chondrophore is 

 a small accessory plate or lithodesma, well developed in Periploma, much 

 smaller or obselete in others; in Periploma, the lithodesma is a calcified, 

 lunate plate lying transversely, in front and above the chondrophore, its 

 function to strengthen or aid in the operation of the resilium; after death, 

 the lithodesma is often detached and lost as the valves open. Interior pearly, 

 the adductor scars and pallial line well marked. The pallial sinus is wide 

 and short. Beaks usually with a seam or crack, fractured by the shrinkage 

 and separation of the resilium and lithodesma. Surface white or cream- 

 colored, and usually minutely granulose. 



Genus PERIPLOMA Schumacher, 1817 



Type species by monotypy, Periploma inaequivalvis Schumacher. Re- 

 cent, West Indies, and the Caribbean. 



Shell suborbicular to elongate-ovate, the beaks opisthogyrate, the pos- 

 terior side typically narrower and shorter. Distinguishing character of the 

 genus is the large chondrophore found in each valve, strengthened below 

 and behind by a rib or clavicle soldered to the wall of the shell. Lithodesma, 

 as described above, is seen only in specimens with tightly closed valves. 

 Surface minutely granulose, most heavily so on the posterior slope, often 



