PANAMIC-PACIFIC PELECYPODA 105 



II. Surface ribbing strongly emphasized. 



Tucetona 



C. Ribs rounded, simple, separated by wide or merely narrowly lined in- 

 terspaces. Subsidiaries lacking or consisting of concentric wrinkles only. 



Ca. Shape subcircular, the submargins not strongly impressed or flattened. 

 Ribs numerous (35-40). 



5. Average size about 38 mm., with high convex umbones and beaks. 

 General color white, lightly or heavily maculated with brown. 



T. multicostata 



Cb. Shape subtrigonal with the anterior and posterior submargins impressed 

 and flattened, the cardinal area narrower. Ribs fewer, not over 30. 



6. Average size about 30 mm. 



T. sirigilata 



7. Average size larger with broader and flatter ribs. 



T. strigUata canoa 



D. Ribs relatively few, large, and covered by strong, cordlike subsidiaries. 

 Shell decidely inequilateral, the beaks opisthogyrous. 



Genus Axinactis 



8. Ribs about 6 in number. 



A. inaequalis 



9. Ribs 10 to 12 in number. 



A. delessertii 



Genus GLTCYSIERIS Da Costa, 1778 



Type species by tautonomy, Glycymeris orbicularis Da Costa, 1778 

 {=zArca glycymeris Linne 1758). Europe. 



Shell usually solid, suborbicular to subcircular, equilateral or nearly 

 so, equivalve, closed, with prominent central umbones ending in small, 

 approximate beaks. Ligament external, covering the cardinal area which 

 may be amphidectic (along both sides of the beak) or entirely anterior, the 

 beaks then becoming opisthogyrate, the cardinal area usually heavily 

 marked with chevron-shaped ligamental grooves. Hinge plate arched, the 

 teeth arranged in a continuous series or the middle zone obliterated by 

 the downward migration of the cardinal area, the lateral teeth always 

 largest. Sculpture principally radial, formed by fine threads, sometimes 

 almost obsolete, the surface then approaching smoothness, or with large, 

 strong ribs. An umbonal ridge running from the beak and angulating the 

 posterior margin is sometimes faintly indicated. Muscle scars distinct, situ- 

 ated high in the valves. Ventral margins of the valve fluted. When fresh, 

 the surface is covered with a dark-colored, soft, velvety periostracum. 



Glycymeris (Glycymeris) OTata (Broderip) Plate 11, figures 6, 6a 



Pectunculus ovatus Broderip, 1832, Proc. Zool. Soc. London, p. 126 Hab. ad insulam 

 Lobos. — Reeve, 1843, Conch. Icon., vol. 1, Pectunculus, pi. 1, fig. 2. 



Pectunculus intermedius Broderip, 1832, Proc. Zool. Soc. London, p. 126 Hab. ad 

 Iquiqui. — Reeve, 1843, op. cit., pi. 1, fig. 1. 



Shell of medium size (length about 44 mm.), obliquely subovate, solid. 

 Surface beneath the brown epidermis white and when well preserved 

 minutely decussate but without the usual fine riblets. Epidermis ashy to 

 dark brown. 



