PANAMIC-PACIFIC PELECYPODA 461 



faint or absent elsewhere. Surface with plain sculpture of growth lines, or 

 with concentric undulations as in Cyathodonta, or strongly radially ribbed. 



The genus Periploma is divided into three subgenera as follows: 



I. Surface plain, unsculptured. 



Subgenus Periploma, s.s. 



II. Surface with concentric or radial ribbed sculpture. 



1. Surface with concentric riblets. 



Subgenus Halistrepta (not regional) 



2. Sculpture of a few, large, strong ribs ending in clawlike spurs at the 

 ventral margin. 



Subgenus Albimanus 



Eight species of Periploma have been described from the Panamic- 

 Pacific region of which five only have been recorded from Panama south- 

 ward. P. planiuscula is the most common form and ranges from southern 

 California to northwestern Peru. The species may be arranged in key form 

 as follows: 



I. Surface of the left valve bearing five, large, radial ribs, the ends of 

 which project like fingers or claws at the ventral margin. 



P. {Albimanus) pentadactylus 



II. Surface of both valves smooth except for concentric lines of growth and 

 minute, submicroscopic granulation. 



A. Valves inequilateral, the beaks being decidedly posterior in position. 



1. Valves subovate to broadly elliptical in shape, the right valve more 

 convex. Open sandy beaches. 



P. {Periploma) planiuscula 



B. Valves subcircular, more nearly equilateral, with the beak and umbone 

 submedian. 



Ba. Granulation in radial lines. 



P. {Periploma) stearnsii 

 Bb. Granulation along concentric lines. 



2. Valve rather large (50 mm,), the posterior side expanded, and well 

 rounded. 



P. {Periploma) aha 



3. Similar but smaller, the posterior side less expanded. 



P. {Periploma) carpenteri 



4. Valves nearly circular, the dorsal margin straight, the posterior end 

 slightly beaked. 



P. {Periploma) discus (PI. 82, fig. 4) 

 Bbb. Dorsal margin straight, the posterior end more or less beaked. 



5. Valves nearly circular. 



P. {Periploma) discus (PI. 82, fig. 4) 



6. Shell smaller, thin, the granulations minute. 



P. {Periploma) lagartilla 



Periploma (Periploma) alta (C. B. Adams) Plate 82, figures 1-lb 



Anatina alta C. B. Adams, 1852, Ann. Lyceum Nat. Hist. New Yoric, vol. 5, pp. 518, 

 547, No. 497.— Turner, 1956, Occas. Papers on MoUusks, Mus. Comp. Zool., 

 vol. 2, No. 20, pp. 30, 31, pi. 19, figs. 19, 20. 



Shell suborbicular, anteriorly subangular and flexuous, otherwise with 



