76 AXEL A. OLSSON 



10. Shell with a solid built, subrhomboidal to subtrigonal shape with 

 prominent umbones often bordered by a sharp, posterior, umbonal 

 angle, the posterior side hence appearing as if sharply truncated. 



Genus Noetia 



11. Shell thinner, modioliform, the posterior side much expanded. Riblets 

 small. 



Genus Sheldonella 

 Subfamily ARCDfAE 



Genus ARCA LInn6, 1758 



Type species defined by Opinion 189, International Commission on 

 Zoological Nomenclature, Area noae Linne. 



Shell of moderate size, subelongate, obscurely winged and generally 

 with an umbonal angle and sharp beaks placed above a wide, flattened 

 cardinal area. Middle of disk crossed by a wide depressed zone which more 

 or less emarginates the ventral margin to form an open byssal gap. Sculp- 

 ture radial, formed by alternating and generally irregular ribs. Cardinal 

 area usually large, as long as the greatest length of the shell, wide and 

 covered by the ligament, marked by scattered tent-shaped grooves radiating 

 from the vicinity of the beaks. Hinge long and narrow, the teeth small and 

 numerous, the posterior set the longest. Periostracum usually heavy and 

 coarsely pilose. 



Area (Area) paciflca (Sowerby) P'late 4, figures 2, 2a-2d; 



Plate 86. figure 6 



Byssoarca pacifica Sowerby, 1833, Proc, Zool. Soc. London, p. 17. 



Area pacifica (Sowerby), Reeve, 1844, Conch. Icon., Area, pi. 11, fig. 75.— Maury, 1922, 



Paleont. Amer., vol. 1, No. 4, pi. 4, pi. 1, fig. 15. 

 Area (Area) pacifica (Sowerby) Hertlein and Strong, 1943, Zoologica, vol. 28, pt. 3, 



p. 155.— Reinhart, 1943, Special Paper, Geol. Soc. America, No. 47, pp. 26, 27, 



pi. 14, figs. 3, 4. — Rost, 1955, Allan Hancock Pacific Expeditions, vol. 20, No. 



2, p. 179. 

 This species is distinguished from its allied West Indian and Caribbean 

 A. zebra Swainson (A. occidentalis Philippi) by its much wider, flatter 

 cardinal area covered by a dark-brown to black ligament; by its generally 

 more distorted form; wider, more expanded posterior side; by its larger 

 byssal gap and coarser sculpture. Judging from specimens we have from 

 Bocas Island, Panama, A. pacifica is apparently also living along parts of 

 the Caribbean coast of Central America. Full grown specimens are usually 

 heavily encrusted with marine growth. The surface of the shell below the 

 periostracum is colored a rich mahogany brown or marked with zigzag strips 

 of brown on a light-colored or white base. 



Length 65.4 mm., height 32 mm., diameter 46.3 mm. Punta Patilla, 

 Panama City. 



Range — Lower California to northern Peru and the Galapagos Islands. 

 Panama: Panama City; Biicaro; Burica Peninsula; Pearl Islands. Ecuador: 

 Esmeraldas; Charapota; Manta; Santa Elena. Peru: Tumbez; Boca Pan; 

 Mancora; Lobitos; Paita. 



Area (Area) nmtabllls (Sowerby) Plate 4, figures 3-3b; 



Plate 6, figures 2-2d 



Byssoarca mutabilis Sowerby, 1833, Proc. Zool. Soc. London, p. 17. 



