334 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



take two or three mating seasons to build up their population to the 

 point where they begin to spread for more room. Fortunately, our 

 leading natural history museums are in constant touch with thousands 

 of mollusk amateurs and naturalists in the United States. From past 

 experiences we know that any "new species" of moUusks discovered by 



FiGUEE 1. — The distribution of the Pomatiopsis snail, a relative of the Oriental 

 carrier of the blood-fluke disease, is limited to the eastern third of the United 

 States. Black dots show the locality records for Pomatiopsis lapidaria. 



amateurs are promptly sent to professionals for naming. In this way 

 any molluscan stranger is likely to come to the attention of malacolo- 

 gists aware of the danger. 



SUMMARY OF MOLLUSCAN INTERMEDIATE HOSTS 



In order to avoid belaboring this article with scientific names, a 

 table of human trematode diseases and their molluscan intermediate 



