vi PREFACE. 



necessary, and most of the new matter refers to minor 

 points. To this there is one exception. I have added 

 to the account of the structure of the oyster a section 

 upon its peculiar fitness for gathering up the germs of 

 cholera and typhoid fever and transmitting them to 

 man, since the importance of clear ideas upon this sub- 

 ject increases with the growth of the cities and towns 

 upon the shores of the Bay and its tributaries, and with 

 the increasing danger of the pollution of the oyster area 

 by sewage. The section upon this important subject is 

 the most notable addition to this edition. 



JOHNS HOPKINS UNIVERSITY, 

 March 25, 1905. 



