PANAMIC-PACIFIC PELECYPODA 77 



Area mutahilis (Sowerby) Reeve, 1844, Conch. Icon., vol. 2, Area, pi. 13, fig. 85. — Maury, 

 1922, Paleont. Amer., vol. 1, No. 4, p. 167, pi. 1, fig. 5.— Maxwell Smith, 194»-, 

 Panama Shells, fig. 648, 



Area (Area)mutabilis (Sowerby), Reinhart, 1943, Special Paper, Geol. Soc. America, 

 No, 47, p. 26, pi. 11, figs. 8-10. — Hertlein and Strong, 1943, Zoologica, vol. 28, 

 pt. 3, p. 154. — Rost, 1955, Allan Hancock Pacific Expeditions, vol. 20, No. 2, 

 p. 180, pi. 11, figs. 1, 2; text-figs. 79, 80 a-c— Hertlein and Strong, 1955, Bull. 

 Amer. Mus. Nat. Hist., vol. 107, art. 2, p, 171. 



Shell generally small, oblong, subrhomboidal, ventricose, often irregular 

 or distorted, the ventral margin carrying a fairly large byssal gap, Umbones 

 wide, often worn, and ending in slightly inrolled beaks and bordered pos- 

 teriorly by a sharp umbonal ridge. Cardinal area elongate, subrhomboidal, 

 high, concave, colored uniformly brown, with a few triangular or lozenge- 

 shaped ligament grooves on the anterior portion. Sculpture consists of fine, 

 more or less scabrous riblets over the main portion of the surface except on 

 the posterior slope where they are much coarser. The periostracum when 

 well preserved is quite dense, yellowish-brown in color and along the um^ 

 bonal keel forms a high, deeply serrated fringe. 



This species is similar to Area umbonata Lamarck of the Atlantic and 

 Caribbean but seems smaller, the largest specimen seen having a length 

 of about 40 mm. 



Range — Gulf of California to Ecuador. Panama: Panama City; Bucaro; 

 Ocones; Burica Peninsula, Ecuador: Puerto Callo; Isla la Plata; Santa 

 Elena. 



Area (Area) trnncata (Sowerby) Plate 4, figure 1 



Byssoarea truneata Sowerby, 1832, Proc. Zool. Soc. London, p. 19. 



Area truneata (Sowerby), Reeve, 1844, Conch, Icon., vol. 2, Area, pi. 11, fig. 74. — Maury, 

 1922, Paleont. Amer., vol, 1, No, 4, p, 169, pi. 1, fig. 2. 



This is a Galapagos Island species, unknown elsewhere. Distinguished 

 from A. pacifica by its longer, more rectangular form, uniform dark-brown 

 color with obscure markings on the sides only, and with a narrower byssal 

 gap. The type lot in the British Museum (Nat. Hist.) consists of three 

 double-valve specimens. Photographs of the type in the British Museum 

 are presented on Plate 4. 



Range — Galapagos Island. 



Genus LITHAKCA Gray, 1842 



Type species by subsequent designation. Gray, 1847, Byssoarea Hth- 

 odomus Sowerby, 



A rock borer. The shell in the adult reaches a length of nearly 100 mm,, 

 elongately cuneiform in shape, subequivalve and strongly inequilateral, 

 the anterior side being much longer, appressed and rounded at the end, the 

 posterior side much shorter, with an angled or carinated umbonal ridge. 

 There is a narrow byssal gap along the middle section of the ventral margin, 

 the byssus itself being a narrow, wedge-shaped plug of a greenish-black 

 color by means of which the shell is attached to one wall of its bore. The 

 beaks are probably prosogyrate in the young but in all the specimens seen 

 the umbones are deeply worn. The cardinal area is long, unsym- 

 metrical wedge-shaped, highest under the beak, narrowed anteriorly and 



