THE RATING OF SHELLFISH 



149 



is indicated by o and the lowest (worst) by 500, represented in 

 tabular form by the following possible results of two analyses : 



Example A 



Oysters 



Numerical 

 Value 



Total rating for B. coli = 



Example B 



The (+) mark means that gas formation in the lactose bile tubes took place, 

 indicating contamination with the colon bacillus. 



The (o) mark indicates that no gas formation took place in the lactose bile tubes. 



The results above indicated are, however, not generally ob- 

 tained in practice. The important question is at what rating 

 shall the shellfish be pronounced unfit for human use, or rather 

 what rating shall be the danger signal as to the quality of this food? 

 There seems to be no uniformity of opinion as regards this point. 

 Thus far, the Bureau of Chemistry has barred oysters from inter- 

 state shipment which gave three positive tests out of five in o.io 

 cc. quantities of oyster liquor, which standard is also adopted by 



