24 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



living ill the mantle cavity of molluscs, and some others) will prove 

 of much interest from the point of view of the present paper. 



Parasitic plants have never been used, so far as I can learn, in such 

 studies as those in which we are here interested, though they present 

 a great mass of usable host-parasite data, but in all the groups which 



Fig. I. — The Atlantic Ocean and the adjacent land areas. The dotted lines indi- 

 cate 2000 fathoms depth. (Modified from a map by W. & A. K. Johnston.) 



furnish these data much further study is needed. A good degree of 

 specificity lietween host and parasite is a desideratum and this we 

 find in a good many cases. 



The rusts are very favorable in some regards. Most of them are 

 restricted in their hosts, many cause lesions which can readily be 

 recognized, as, for example, the Peridermiums of pines and the branch 

 " nests " of cedars. Many of the rusts of the conifers produce distor- 

 tions in the hosts which could be identified in fossils. The two hosts, 



