336 AXEL A. OLSSON 



the Panamic-Pacific zone but it is reasonable to expect that eventually one 

 or more species will be discovered there. E. californica Dall, 1916 is said 

 to range from San Pedro, California, to Magdalena Bay, Lower California. 



Superfamily TELLINACEA 

 Family DONACIDAE 



The shell is generally transversely trigonal to cuneiform, equivalve, 

 solid in texture, with entire or crenulated and usually closed margins. The 

 surface is smooth or with fine radial lineation or riblets. The ligament is 

 external, short, usually opisthodectic, and attached to and along a nymphal 

 plate. Pallial line distinct, placed a short space above the margin, its sinus 

 short or of medium length, generally directed horizontally, its end well 

 rounded. The adductor scars are subequal in size and often deeply im- 

 pressed. Substance of the shell is usually coarse, porcellaneous, white ex- 

 ternally or grading into violet or purple, the same coloration in the interior. 



There are two genera in the Panamic-Pacific region. 



I. Shallow-water and beach shells, of small or medium-sized (seldom 

 above 45 mm.), cuneate to trigonal in shape, the posterior side shorter, 

 the posterior slope rounded, flattened, and sometimes sharply trun- 

 cated. Surface smooth but nearly always showing fine radial lines, the 

 ventral margins of the valves typically crenulated. 



Genus Donax 



II. Lagoonal, the valves often quite large, the posterior side cuneate but 

 not flattened or truncated. Surface smooth or roughened slightly by 

 concentrics; radials are absent or merely in the shape of minute lines, the 

 valve margins smooth. Surface covered by a thin or coarse, straw 

 yellow periostracum. 



Genus Iphigenia 

 Genus DOJfAX Linn6, 1758 



Type species by subsequent designation, Lamarck, 1799, Prodrome, 



D. truncvlus Linne; or by Gray, or Hermannsen 1847, as D. rugosus 



Linne. 



Shell usually solid, porcellaneous, smooth and polished, elongately 

 subtrigonal or wedge-shaped, the anterior side longer and rounded at the 

 end, the posterior side shorter, with a straight or subtruncated margin. The 

 ligament is external and shows as a short, rounded or convex, brown knob 

 behind the beaks. The hinge has two cardinal teeth in each valve, the 

 lateral teeth are more variable but usually there are two strong lateral 

 teeth in the left valve and their corresponding sockets in the right. Surface 

 smooth, glossy, often showing a fine sculpture of small radial lines or low 

 Tibs due mostly to a radial structure within the outer layer and which gives 

 rise to strong crenulations along the ventral margin; divisions between the 

 radials may be simple lines, or wider, deeply pitted grooves; fine con- 

 centrics may also be present, heavier over the posterior submargins. The 

 periostracum is thin, inconspicuous or wholly wanting. Pallial sinus is wide 

 and deep, horizontally directed, its end well rounded. 



Donax trunculus has been accepted as the type species of the 

 genus; a procedure which will be objected to by students who do not con- 

 sider the names in Lamarck's Prodrome as constituting type designations. 

 Several names have been proposed as subgenera for Donax but with the 



