320 AXEL A. OLSSON 



Superfamily MACTBACEA 

 Family MACTRIDAE 



The shell is ovate, subtrigonal or hatchet-shaped, equivalve and gen- 

 erally inequilateral, usually convex, and of thin or coarse texture. The 

 posterior-dorsal area is usually well defined, set apart by a line, angle or 

 by a high fringe or keel. The ligament is almost completely internal, the 

 major part or resilium lodged in a deep, cup-shaped pit or chondrophore 

 in the hinge plate; the tensilium section of the ligament is always small 

 and it may show externally behind the beaks as a small, brown knob, or 

 it may lie concealed below the hinge margin as in Mulinia. The hinge plate 

 is usually wide and carries the large, deep chondrophoral pit or resilifer 

 in the middle bordered on the anterior side by an inverted V-shaped cardinal 

 tooth and on the two sides by the lateral teeth or their sockets. The um- 

 bones are usually high, convex, and prominent and terminate in small, 

 adjacent, prosogyrate beaks. In the interior, the adductor scars are sub- 

 equal and generally show distinctly. Pallial sinus large, rounded or squared- 

 off at the end. The inner ventral margins are smooth. Surface smooth or 

 marked with concentric growth lines; sometimes the surface is waved or 

 deeply undulated, and overrun with coarse, vermiculate striae, generally 

 radial. Color white, the periostracum thin or coarse, yellow, gray, brown, 

 or nearly black. The siphons are united to the tips. 



The Mactridae are best divided into genera on the characters of 

 the ligament, supplemented by the external features of the shell. The 

 external section of the ligament or tensilium is always small and in the 

 majority of forms, it is attached to a small scar set along the dorsal margin 

 just behind the beaks and in living specimens, it is visible externally. In 

 Mactrellona (Mactrella), the tensilium is attached to the dorsal surface of 

 a small, nymphal plate which is connected anteriorly with the margin of 

 (the valve extended below the beak. In Mactra, s.s., the tensilium lies 

 deeper and only a small part may show above the margin, the scar of its 

 attachment separated from the resilifer or chondrophoral pit by a small 

 ridge, sometimes reduced to a mere ridge. In Mtdinia, the entire ligament is 

 internal, no part of which shows externally, but the tensilium is still 

 functional and its scar of attachment is placed in the roof of the chondro- 

 phore but clearly separated from the main portion of the resilifer by an 

 open or free space. In Rangia, the ligamental features are the same as in 

 Mulinia. 



As a family, the Mactridae are especially well defined and in general 

 easily distinguished from most other marine clams by their characteristic 

 hinge and by their large, internal ligament attached to a deep, spoon- 

 shaped cup or chondrophore in the middle of the hinge plate. In key form, 

 the Panamic genera have been arranged as follows: 



Key to Panamic-Pacific genera 



I. Shell hatchet-shaped, ovate-trigonal, the umbones submedian in posi- 

 tion, with small, prosgyrate beaks, the two ends of the valves more 

 or less alike, the zone of greatest convexity lying along the umbonal 

 slope. The posterior-dorsal area is usually well defined, often set off 

 by an angle or keel. 



Subfamily Mactrinae 



