iv PREFACE TO SEVENTH EDITION. 



contains those procedures which have been tried and found 

 trust worthy. 



In the descriptive section certain less important species 

 and varieties have been eliminated, and those of more 

 frequent occurrence substituted. 



Nomenclature has been altered to conform to the 

 suggestions of the advanced systematists. 



In presenting this edition it gives me great pleasure 

 to acknowledge the valuable assistance rendered by 

 Prof. D. H. Bergey, of this Laboratory. 



A. C. A. 



LABORATORY OF HYGIENE, 

 UNIVERSITY OF PENNSYLVANIA. 



