PANAMIC-PACIFIC PELECYPODA 337 



exception of Machaerodonax and Amphichaena, the others are not appli- 

 cable to the species here under consideration. 



Species of Donax are generally most plentiful on a sand-bottom en- 

 vironment, and many of them are typically shore shells, burrowing in the 

 sand between tide levels along open beaches exposed to a heavy, pounding 

 surf. Certain species are collected in large quantities and sold in the markets 

 of Central America and South America under the name of "Almejas" and 

 used in the preparation of an excellent light soup or "caldo". 



Subgenus DONAX, s.l. 

 The shell is elongately trigonal to ovately trigonal in shape, porcel- 

 laneous, solid, with high umbones, usually smooth and glossy but often 

 showing a fine radial lineation or ribbing which end in strong crenulations 

 along the ventral margin. The margins of the valves are closed tightly. 



Subgenus MACHAERODONAX Romer, 1870 



The shell is elongate, razor-shaped, the anterior side long, the much 

 shorter posterior side set apart by a sharp keel. The margins of the valves 

 are open or gaping at both ends. 



Key to Panamic-Pacific species of Donax 



I. Shell open or gaping at both ends. Subgenus Machaerodonax 



1. Shell elongated, razor-shaped, the anterior side much the longer, with 

 the dorsal and ventral margins straight and parallel, the anterior end 

 obliquely rounded. Posterior side short and with a sharply angled 

 umbonal keel. Surface smooth, polished, and rayed. 



D. transversus 

 II. Shell margins closed all around. Subgenus Donax, s.l. 



A. Shape of shell much elongated, its length more than twice its height, 

 the beak and umbone placed near the middle line. 



a. Shape more or less tageloid, the substance of shell relatively thin. 



2. Posterior side high and flaring, its surface depressed or weakly vaulted, 

 with evident radial sculpture and a rayed color pattern. 



D. culter 



b. Shape not tageloid, the posterior side narrowed, produced into a 

 rounded end. Shell rather solid, the radials small and indistinct. 



3. Ratio of height to length about 1 to 2.5. Surface plain. 



D. gracilis 



4. Shorter, ratio about 1 to 2.3. Color white with the dorsal margins 

 marked with purple. 



D. naviculus 



B. Shell subtrigonal to elongate, its length generally less than twice the 

 height. 



BA. Posterior side strongly flattened or truncated, setoff by a sharply 

 angled umbonal keel. 



c. Shell high, subtrigonal, the beaks and umbones subcentral. 



ca. Posterior slope with coarse sculpture, divided near the middle by a 

 deep, radial groove terminating at the margin in a sharp tooth. 



5. Shell high and broad, thinner than the next. Coloration varied. 



D. dentiferus 



cb. Posterior sculpture strong, uniform, no tooth at the margin. 



6. Shell often large (30 to 42 mm.), solid. Generally white, the interior 

 irregularly stained with purple. 



D. asper 



