322 AXEL A, OLSSON 



for the family. Posterior-dorsal areas usually distinct. Siphons partly or 

 wholly naked and retractible into the shell, the mantle lobes separated 

 ventrally between the siphons and the anterior adductor. 

 Genus MACTRA Linn6, 1767 



Type species by subsequent designation, Gray, 1847, Mactra stultorum 

 Llnne. 



The shell is usually elongately ovate, coarse in texture, the ends sub- 

 equal or the posterior side may be a little longer. The posterior-dorsal area 

 is low, flattened or rounded, and set apart weakly by a small line or ridge. 

 The external portion of the ligament is small and attached to a scar along 

 the dorsal margin just behind the beak. Surface smooth except for the 

 usual lines of growth but often showing small waves or concentric undula- 

 tions over the beaks. 



Species belonging to the typical section of the genus Mactra are not 

 represented in the Panamic region. 



The genus has been divided into several smaller groups or subgenera 

 but the lines between these groups are not often well defined. In this work, 

 the Panamic species are referred to three subgenera defined as follows: 



I. Valves subequilateral, the two ends appearing almost alike, the beaks 

 and umbones near the middle. Surface of beaks and umbones generally 

 rippled, the undulations strongest on the side slopes, fading away 

 below. 



1. Shell relatively small and of a medium weight. 



Subgenus Micromactra 

 II. Valves strongly inequilateral, generally thick-walled. Surface of beaks 

 and umbones not sulcated. Clams of moderate or large size. 



2. Posterior side much longer than the anterior, the adductor scars un- 

 equal, the posterior one the larger. The anterior arm of the right 

 cardinal tooth lies above or in the same general plane as the lateral 

 lamina. 



Subgenus Mactroderma 



3. Two ends of shell of nearly the same length, the anterior side narrow 

 and cuneiform. Adductor scars subequal. Anterior arm of the right 

 cardinal tooth lies below the plane of the lateral lamina. 



Subgenus Mactromeris 



Subgenus MACTR0DER3U Dall, 1894 



Type species by original designation, Mactra velata Philippi. 



Shell inequilateral, the posterior side longer than the anterior, convex, 

 and thick-walled. Posterior scar the larger. Posterior-dorsal slope rounded, 

 set apart by a small line. Anterior arm of right cardinal tooth continuous 

 wirh or in the same plane as the lateral tooth. Surface crude, marked with 

 coarse, uneven growth lines, and covered by a coarse, brown to black 

 periostracum. 



Hactra (Mactroderma) relata Pliilippi Plate 58, figures 1-lc 



Mactra velata Philippi, 1848, Zeitschr. fiir Mai., p. 153.— Philippi, 1850, Abbild. und 

 Beschreib. Conchylien, vol. 3, p. 137, pi. 3, fig. 5. — Reeve, 1854, Conch. Icon., 

 vol. 8, Mactra, pi. 5, fig. 20. 



Mactra (Mactroderma) velata Philippi, Dall, 1909, Proc. U.S. Nat. Museum, vol. 37, 

 No. 1704, p. 274. 



This is the common, coastal Mactra from Peru northward to Panama, 

 recognized by its large, coarse shell. Philippi's original figure illustrates an 



