164 AXEL A. OLSSON 



higher, coarser ribs, and the umbones are much more inflated. Northern 

 shells which we have seen, are thinner than from the south, with narrower 

 ribs, and take on the general characteristics of A. aequistdcatus. 



Range — Lower California southward to northern Peru. Panama: Pearl 

 Islands; Garachine; San Carlos; Bucaro; Panama City. Canal Zone: 

 Venado Beach; Palo Seco; Fort Amador. Colombia: Gorgona Island; Isla 

 del Gallo. Ecuador: Esmeraldas; Charapota- Jaramijo; Manta; Puerto Callo; 

 Santa Elena; Punta Mambri. Peru: Tumbez; Zorritos; Mancora; Negritos; 

 Bayovar. 



Subgenus PACIPECTEJf, new subgenus 



Type species Aequipecten tumbezensis (d'Orbigny). 



Shell with broadly circular, slightly convex, subequal valves, with 

 rounded, flat-topped or triangular ribs, their interspaces as wide or wider. 

 Surface of the ribs and interspaces smooth except for growth-line concentrics 

 (sometimes coarsely developed around the margins of old shells) and 

 finer, indistinct radial threads or striations. Valves dissimilar in 

 color, the left valve much darker, the right usually white. Ears 

 large, the posterior one spreading, the right anterior long and narrow and 

 cut into by a deep byssal notch bordered by a few pectinate teeth. Hinge 

 line straight, the resilifer or chondrophore large and bordered by elongated 

 crural ridges on each side, the space above the crurae furrowed and finely 

 wrinkled. Internal margin of shell fluted by the external ribs. 



The species of this subgenus have been referred to Leptopecten from 

 which they differ by their more equilateral and heavier valves. As fossil, it 

 is represented by A. nelsoni Olsson from the upper Miocene, Tumbez beds 

 of Peru. 



Aequipecten (Pacipecten) tumbezensis (d'Orbigny) Plate 21, figures 2-2c 



Pecten aspersus Sowerby, 1835, Proc. Zool. Soc. London, p. 110. Tumbez. — Sowerby, 



1843, Thes. Conch., vol. 1, Pecten, p. 51, pi. 19, figs. 198, 199. (Not of Lamarck, 



1819). 

 Pecten tumbezensis d'Orbigny, 1846, Vol. Araer. Merid., vol. 5, p. 663. (New name for 



P. aspersus Sowerby, not Lamarck). — Maxwell Smith, 1944, Panamic Marine 



Shells, p. 52, fig. 692. 

 Pecten soiverbyi Reeve, 1852, Conch. Icon., vol. 8, Pecten, pi. 1, fig. 4. (New name for 



P. aspersus Sowerby). 

 Pecten (Leptopecten) tumbezensis d'Orbigny, Hertlein, 1935, Proc. California Acad. 



Sci., ser. 4, vol. 21, No. 25, p. 314, pi. 19, figs. 11, 12. — Hertlein and Strong, 



1946, Zoologica, vol. 31, pt. 2, p. 60. — Hertlein and Strong, 1955, Bull. Amer. 



Mus. Nat. Hist., vol. 107, art. 2, pp. 180, 181. 

 Pecten cf. latiauritus fucicolus Dall; P. latiauritus splendens Li; P. latiauritus indentus 



Li, Pilsbry, 1931, Proc. Acad. Nat. Sci. Philadelphia, vol. 83. p. 429. 



Shell small or of medium size, seldom above 35 mm. in height, sub- 

 equivalve. Ribs number from 13 to IS; those of the right valve are nearly 

 square in section between deeply grooved interspaces; those of the left 

 valve narrowly triangular between wider interspaces. Surface is nearly 

 smooth except for minute concentrics and still finer radial striae. The 

 right valve is usually light-colored, white, but many specimens show a 

 blotch of brown along posterior submargins and occasionally even in the 

 middle; the left valve is colored dark-brown, reddish-brown to slaty black, 



