PAN'AMIC-PACIFIC PELECYPODA 323 



abnormal specimen with a high, rounded, posterior side. The species is 

 common in Sechura Bay, Peru, where it grows larger than any specimens 

 seen from further north. The shell is variable in shape, often high, and 

 trigonal. 



Length 115 mm., height 80 mm., diameter of a left valve 23.2 mm. 

 Bayovar, Peru. 



Range — Lower California (Dall) to northern Peru, and the Galapagos 

 Islands. Panama: San Carlos; Panama City. Panama Canal Zone: Venado 

 Beach. Ecuador: Sua; Padernales; Manta. Peru: Caleto Sal; Lobitos; Paita; 

 Sechura Bay; Lobos de Tierra. 



Subgenus MACTR03IERIS Conrad, 1S68 



(Simomactra Dall, 1894, type species, M. dolabriformis Conrad.) 

 Type species by subsequent designation, Dall, 1898, Spisula ovalis 

 Gould {=Mactra polynyma Stimpson). 



Shell inequilateral, the two ends are of nearly the same length but 

 differ greatly in shape, the posterior side is wide and round while the 

 anterior side is narrow and cuneiform. The adductor scars are small and 

 subequal. Hinge of medium size, the lateral teeth spaced about the same 

 distance from the middle, the right, anterior lateral tooth not adjacent or 

 in the same line as the anterior arm of the cardinal tooth. 



Mactromeris has usually been considered a section or subgenus of 

 Spisula or of Hemimactra, said to differ from those genera mainly in that 

 the surface of the lateral teeth is smooth or without striations; in my 

 opinion, the affinities of Mactromeris to Mactra seem equally as close. The 

 typical species of the subgenus live in northern waters. 



Mactra (Mactromeris) dolabriformis (Conrad) Plate 57, figures 1. la; 



Plate 58, figures 4, 4a 



Spisula dolabriformis Conrad, 1867, Amer. Jour, Conch., vol. 3, pt. 2, p. 193 Panama. — 

 Conrad, 1869, Araer. Jour. Conch., vol. 5, p. 108, pi. 12, fig. 1 (only). — Burch, 

 19+5, Min. Conch. Club. South. Calif., No. 44, pp. 17, 18. 



Mactra dolabriformis (Conrad), Dall, 1894, Nautilus, vol. 7, p. 138, pi. 5, fig. no. 1. — 

 Maxwell Smith, 1944, Panamic Marine Shells, p. 67, fig. 865. 



Shell trigonal-ovate or flattened cuneiform, inequilateral, the anterior 

 side is narrower and somewhat longer, the posterior side higher and well 

 rounded, thin or of medium weight. Umbones submedian, flattened, ending 

 in small adjacent beaks which touch the hinge margin. Surface smooth or 

 marked with fine, hairlike growth lines and covered by a thin, straw- 

 colored, deciduous periostracum. Pallial sinus well defined but short, the 

 adductor scars subequal. 



A right valve from Atacames, Ecuador, measures: length 93 mm., 

 height 55 mm., diameter 13 mm. 



A rare species but apparently of wide distribution through the Panamic 

 faunal province. In shape similar to Spisula falcata (Gould) but generally 

 smaller, thinner, and more depressed. A thin but distinct shelly ridge lies 

 between the external ligament and the chondrophore. 



Range — Southern California to Panama and Ecuador. Mexico: Nino 

 Bay, Sonora Mexico (H. N. Lowe, ANSP). Panama: Panama (Conrad, 



