282 Our Food Mollusks 



or hole. By the size and number of these pits, the dig- 

 ger may judge whether his labor is likely to be worth 

 while. 



Such a bottom is well illustrated by Figure 54, where 

 individuals happen to be very numerous. Though most 

 of the pits are relatively large, one cannot always be cer- 

 tain that all clams forming them are also large, for often 

 a pit is used by several individuals in common. These 

 lie at different depths according to size, the larger occu- 

 pying the lower levels. This distribution allows an im- 

 mense number of clams to crowd together where there is 

 food enough to support them all. The clams shown in 

 Figure 55, for example, were all dug from beneath a 

 single square foot of a prolific bed. 



The bottom in which clams burrow is not always 

 muddy. Most often it is made up largely of sand mixed 

 with mud or clay, and at times they are found in al- 

 most clear sand. In the latter case the shells, instead of 

 being discolored as in muddy bottoms, are pure white. 

 In some localities, usually on sandy beaches, lime is not 

 abundant, and shells are thin. Such clams are known as 

 " paper shells." 



When a clam bed is dug frequently, many are killed 

 by the fork or hoe, and subsequently become covered. 

 Their decaying bodies stain the surrounding sand a 

 pitchy black, and give it an offensive odor, so that clam 

 digging is not always a pleasant occupation. If clams 

 are very closely crowded, especially where tide currents 

 are sluggish, the decay of several bodies leads to the 

 death of others, indirectly by the formation of com- 

 pounds that eat through the lime of the shells, and ap- 

 parently also directly by poisoning or infection. 



Mya sometimes lodges and burrows in places quite in- 



