— 10 — 



Fig. 5. Miitilus calif ornianus. One-half 

 natural size. 



cc. Umbo near, but not at. extreme entl; shelLs frequently bearded; brown, usually 

 solitary, in mud or gravel. 



Modiolus 



p. 27 ; pi. 4 , fig 3 



Fig. 6. Modiolus rectus. One-half natural size. 



bb. Adductor muscles equal or nearly so ; umbo not at end of valve in forms here 

 treated (except Solen) ; mantle edges fused in at least two places forming 

 two siphonal apertures, which are usually prolonged into siphonal tubes ; 

 adults without byssal attachment ; usually capable of moving about by 

 means of the foot ; all capable of burrowing to some extent and habitually 

 doing so ; color of shells various ; never black. 



True clams 



c. Valves closing completely, or at least wilh no marked gapuig, siphons com- 

 pletely retractile and not visible in the closed shell. 

 d. Shells thick and strong, animal symmetrical with similar valves, siphons 

 usually united and short ; if burrowing not lying on one side, 

 e. Siphons extremely short, not projecting beyond shell when extended ; no 

 pallial sinus; not burrowing deeply. (See Fig. 7; compare with 

 Fig. 8). 



Cockles Curdinm 



Fig. 7. Cardiiiiii cor- Fig. 8. Donax laevigata, 



bis. One - half natural One and one-half times 



size. natural size. 



f. Radiating ribs conspicuous and ."sharply raised above surface, 

 g. Ribs roughened. 



C. co7-his 



p. 28; pi. 5 , fig. 2 

 gg. Ribs with rows of spines. 



C. quadrlfjenayium 



p. 28; pi. .5 , flg. 1 

 ff. Ribs faint and gently rounded. 



C. datum 



p. 29 



