PANAMIC-PACIFIC PELECYPODA 325 



Shell elongate, ovate, white, covered with a thin, gray-brown perio- 

 stracum, depressed to moderately convex. The posterior side is a trifle 

 longer than the anterior, both ends evenly rounded. The posterior gap is 

 large, the anterior one small. The surface of the extreme tip of the umbones 

 are smooth or finely undulated with concentric waves, deepest on the sides; 

 elsewhere, the surface is smooth or marked with fine, hairlike concentrics 

 only. The posterior-dorsal slope as in some species of Mactroderma is 

 sloping. 



Length 50.5 mm., height 32 mm., diameter 17.2 mm. An average 

 specimen. 



Range — Nicaragua southward to Panama. Nicaragua: Type area from 

 Potosi and Monypenny Point, Gulf of Fonseca (Hertlein and Strong). 

 Panama: Montijo Bay. Panama Canal Zone: Palo Seco. Ecuador: Off Cabo 

 Pasado (Hertlein and Strong). 



Mactra (Micromactra) Tanattae Pilsbry and Lowe Plate 57, figures 5, 8c 



Mactra {Micromactra) vanattae Pilsbry and Lowe, 1932, Proc. Acad. Nat. Sci. Phila- 

 delphia, vol. 84, p. 90, pi. 16, figs. 4-4b. — Maxwell Smith, 1944, Panamic 

 Marine Shells, p. 66, figs. 853, 853a. — Hertlein and Strong, 1950, Zoologica, vol. 

 35, pt. 4, p. 232. 



Shell usually small (length 48 mm., or less), compressed, gaping pos- 

 teriorly and moderately solid, white, under a gray or drab periostracum. 

 The beaks are smooth and plain or sculptured with short, concentric waves 

 in front of the posterior-dorsal ridge. Beaks are nearly median in position, 

 the two ends much alike. Posterior-dorsal slope narrow, defined by a small 

 line or ridge, slightly vaulted in the right valve, more flattened in the left. 

 Pallial sinus short, wide, rounded. 



Length 31 mm., height 21 mm., diameter 11.2 mm. 



In the variety or subspecies M. acymata Pilsbry and Lowe, the undu- 

 lations on the beaks are obsolete. M. vanattae is the common Microtnactra 

 in Panama. 



Range — Guatemala to northern Peru. Panama: Panama City; Burica; 

 Bucaro; San Miquel, Rey Island, Pearl Islands. Ecuador: Galeras; Manta; 

 Santa Elena; Ancon Point; Puno. Peru: Tumbez. 



Mactra (Micromactra) an^nsta Reeve Plate 57, figures 2-2b; 



Plate 85, figure 9 



Mactra angusta Reeve, 1854, Conch. Icon., vol. 8, Mactra, pi. 18, fig. 93 ; Deshayes, 

 1855, Proc. Zool. Soc. London for 1854, p. 67; Weinkauff, 1884, Conchy].— 

 Cab. von Martini-Chemnitz, bd. 11, abt. 2, Mactracea, p. 70, taf. 25, figs. 2, 2a. 



Mactra {Micromactra) angusta Reeve, Hertlein and Strong, 1950, Zoologica, vol. 35, 

 pt. 4, p. 231, pi. 2, figs. 14, 18.— Hertlein and Strong, 1955, Bull. Amer. Mas. 

 Nat. Hist., vol. 107, art. 2, pp. 203, 204. 



Mactra {Micromactra) atacama Pilsbry and Olsson, 1941, Proc. Acad. Nat. Sci. Phila- 

 delphia, vol. 93, p. 73, pi. 14, figs. 1, 3. 



The shell is white, transversely elongate, compressed, gaping slightly 

 at the posterior end. The beaks are nearly median in position, the shell 

 hence nearly equilateral, the anterior side obliquely rounded at the end, and 

 somewhat shorter than the posterior which is bluntly pointed. The anterior- 

 dorsal margin is slightly concave, the posterior one a little arched. Valves 

 are relatively thin, the surface white, often subtranslucent, covered by a 



