— 11 — 



ee. Siphons short or moderate, a pallial sinus present; burrowing more or 

 less deeply, 

 f. Size small, valves crenulate or toothed on inner surface of margins, 

 teeth interlocking in closed shell; siphons separate; pallial sinus 

 reaching about half way from posterior to anterior muscle scar. 

 Wedge shells Donax 



S. Umbo central. 



D. calif ornica 



p. 47; pi. 16, fig. 1 



gg. Umbo much nearer posterior end. 



D. laevigata 



p. 47 ; pi. 16 , fig. 2 



ff. Size moderate or large, valves usually smooth at margins, sometimes 

 roughened but without regular crenulations ; siphons united, 

 g. Pallial sinus deep, reaching more than half way to anterior muscle 

 scar. (See Fig. 9; compare with Fig. 10.) 



h. Valves with radiating ribs. 



Papliia 



Fig. 9. Papliia stami- 

 nea. One-half natural 

 size. 



Fig. 10. Tivela stultorum. One- 

 half natural size. 



i. Valves about round in outline, ribs prominent, concentric ridges 

 faint ; pallial sinus reaching somewhat more than half way 

 to anterior scar ; valve margins rougliened. 



Rock cockle F. staminea 



p. 38 ; pi. 10 , fig. 2 



ii. Valves elongated, ribs small but distinct, concentric ridges more 

 prominent than ribs ; pallial sinus reaching g of distance to 

 anterior muscle scar ; valve margin smooth. 



Thin-shelled cockle P. tenerrima 



p. 38; pi. 10 , fig. 1 



hh. Valves without radiating ribs, concentric ridges distinct ; pallial 

 sinus reaching i of distance to anterior muscle scar. 



Washington clams Saxidomus 



i. Concentric ridges bold and prominent. 



S. nuttalli 



ii. Concentric ridges finer and less conspicuous. 



S. giganteus 



p. 3.5 ; pi. 7 , figs. 1, 2 



p. 35 ; pi. 8 , figs. 1, 2 



?. Pallial sinus small, reaching not more tlian half way to anterior 



muscle scar. (See Fig. 10.) 

 h. Ligament external, valves heavy with moderate or thick edges, 

 i. Valves smooth, very heavy, with marked glossy, persistent perios- 

 tracum; pallial sinus reaching less than one-third way to 

 anterior muscle scxir ; found in clear sand on open beaches. 

 Plsmo clam Tivela stultonim 



p. 29 ; pi. 6 , figs. 1, 3 



