252 ELEMENTS OF WATER BACTERIOLOGY 



responsible for such anomalous results in shellfish 

 examinations. 



COLON BACILLI AND STREPTOCOCCI IN DIFFERENT 

 PORTIONS OF THIRTY CLAMS 



It will be noted from Clark's table above that the 

 shell liquor is not only freer from overgrowths than the 

 portions of the body of the clam, but that the propor- 

 tion of positive reactions is in each case higher. Since 

 the shell water is of course easier to examine than the 

 macerated animal, this is now generally adopted as the 

 standard material for examination. 



Self-purification of Shellfish. In connection with 

 the bacteriological examination of shellfish for colon 

 bacilli certain investigations have been carried out 

 which are of great importance from the commercial 

 as well as from the sanitary standpoint. Phelps (191 1) 

 has shown that oysters which develop in waters sub- 

 ject to sewage pollution may be purified or entirely 

 freed from colon bacilli by the removal of the oysters 

 themselves to waters of purer character, when, after 



