112 THE NAUTILUS. 



the rocks, in order to get down below the hot, dry rocks to where 

 there was a little moisture. 



It seems apparent that the living shells live among the loose stones, 

 in the early spring crawling around over the damp rocks. As the 

 warm spring and summer days approach, the rocks become dry. The 

 snails previously crawling over them cease activity, and instead of 

 all of them crawling for protection under the loose rock, some throw 

 their protective film across the opening of the shell while yet on the 

 rocks ; the sun kills the animal, which dies, leaving the dead shell to 

 bleach and become a conspicuous feature on the rocks. Here the 

 shells remain, very few of them washing any distance. 



All of the living shells taken appeared dead except a few. Invari- 

 ably, however, when a shell sank in water the snail within was alive. 

 It is probable that the species is continued by the hibernation of 

 some of the more fortunate individuals which are deeper in the rocks, 

 where there is more moisture. 



ADAPTATION OF MOLLUSKS TO CHANGED CONDITIONS. 



BY A. C. BILLUPS, LA WRENCEBURG, INDIANA. 



Many years ago the Ohio river at and below Cincinnati, Ohio, was 

 one of the most prolific hunting grounds of the collector of the fresh- 

 water species of mollusks. At that time abounded in immense 

 numbers (as is shown by the large quantities of duplicates in the 

 collections of all the old collectors) Anciilosa praerosa Say. About 

 twelve years ago this shell disappeared almost entirely and for many 

 years not a single specimen was found ; this year, however, a few 

 adults and large numbers of young shells have been found in locali- 

 ties where it is certain that no shells have existed for many years. 

 The explanation of this occurrence is as follows; A. praerosa Say 

 was a long time ago one of the most common of the river snails ; the 

 growth of the city of Cincinnati and the numerous factories along the 

 banks of the river, each and every one adding to the filth of the water, 

 pouring in sewage and acids, have rendered the water so foul that the 

 Anculosa of the old day have been exterminated, all but a few of the 

 most hardy of the species, which probably crawled to the lowest and 

 most inaccessible parts of the river, where they managed to exist and 

 to produce young : the young shells have become more used to the 

 changed conditions, which worked such havoc with their ancestors, 



