32+ AXEL A. OLSSON 



ANSP); Concepcion Beach near Las Tablas; Pedro Gonzalez Id. (Pearl 

 Islands), J. P. E. Morrison, USNM. Ecuador: Atacames. 



Subgenus MJCROMACTRA Dall, 1894 



Type species by original designation, Mactra californica Conrad non 

 Deshayes. 



Shell small (length less than 60 mm.), the beaks and umbones sub- 

 median, depressed to slightly convex, the umbones generally sulcated. Hinge 

 that of Mactrotoma, the scar of the external ligament small, seated on the 

 margin above the chondrophore, not separated by a nymphal ridge. Pos- 

 terior-dorsal area narrow, generally flattened. 



On hinge characters, Micromactra is similar to Mactrotoma, but it 

 differs constantly by the smaller size of its typical species and generally in 

 have the surface of the umbones neatly rippled with concentric undulations. 

 The group is abundantly represented as fossil in the Tertiaries of the West 

 Indian-Caribbean region. 



Key to species of Micromactra 



I. Pallial sinus large, extending to or beyond the mid-x:enter of the valves. 



1. Shell relatively high, the height about 68 to 70 per cent of its length. 



M. isthm,ica 



2. Shell longer, the height less than 65 per cent of its length, relatively thin. 



M. fonsecana 

 II. Pallial sinus shorter, not reaching to the mid-center. 



3. Shell short, the anterior-dorsal margin straight, subsolid. 



M. vanattas 



4. Shell longer, the anterior-dorsal margin concave, relatively thin. 



M. angusta 



Mactra (Micromactra) isthmica Pilsbry and Lowe Plate 57, figure Sd 



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

 delphia, vol. 84, p. 89, pi. IS, figs. 1, 2; pi. 16, fig. 5. 



The shell is broadly ovate-elliptical, depressed convex, widely gaping 

 at the posterior end, slightly so anteriorly, white under a buff to olivaceous 

 periostracum, minutely and delicately thread-striate. The small beaks have 

 concentric waves in front and behind, sometimes continuous across the 

 whole umbonal surface. Posterior-dorsal area as in Mactroderma is wide, 

 with a downward slope and is defined on the lower side by a small thread. 

 Pallial sinus ample, extending past the middle to 56 to 60 per cent of the 

 length. 



Length 55 mm., height 38.5 mm., diameter 20 mm. 



A more solid and higher shell than M. fonsecana. In shape, this shell 

 is much like species of Mactrotoma. 



Range — El Salvador to Panama. El Salvador: Gulf of Fonseca (Pilsbry 

 and Lowe). Panama: near Panama City (J. Zetek). 



Mactra (Micromactra) fonsecana Hertlein and Strong Plate 57, figures 4, 4a, 8b 



Mactra {Micromactra) angusta Deshayes, Pilsbry and Lowe, 1932, Proc. Acad. Nat. 



Sci. Philadelphia, vol. 84, p. 89, pi. 15, figs. 3, 4; pi. 16, fig. 3. Not M. angusta 



Reeve, 1854. 

 Mactra {Micromactra) fonsecana Hertlein and Strong, 1950, Zoologica, vol. 35, pt. 4, 



p. 232, pi. 2, figs. 16, 19, 20.— Hertlein and Strong, 1955, Bull. Amer. Mus. Nat. 



Hist., vol. 107, art. 2, p. 204. 



