100 THE NAUTILUS. 



on the Bahamas, are illustrative examples of how certain snails are 

 thriving under favorable conditions. 



One of the most notable instances of this kind is the following: 

 Bithynia tentaculata, since its introduction from Europe, has rapidly 

 gained ground and is now widely distributed and common, e. g., in 

 Lake Erie. At the waterworks of Erie, Pa., where the intake is 

 four or five miles out in the lake, about one mile outside of Presque 

 Isle, the " wells at the pumping station are periodically filled up with 

 these snails, which have to be taken out by the wagon loads. And 

 they are also driven through the pipes over the city, and often plug 

 up faucets." Examination showed that almost all are dead shells of 

 Bithynia, with occasionally a Planorbis or Physa mixed in. The 

 same trouble may be experienced at other waterworks along the lake, 

 yet the first engineer of those at Cleveland told me that they had no 

 difficulty of this kind so far. 



It is well known that canals have been excellent habitats for mol- 

 lusks, and richly populated, especially with Uniom'da and Sphariida, 

 and were highways of migration from drainage to drainage for many 

 species. The more it is deplored by the conchologist that many of 

 them have been neglected and abandoned of late years. 



To come back to land snails, it may be said that in Europe a large 

 number of them have become regular habitants of walls, yards, gar- 

 dens, vineyards, orchards and fields, much more so than in most 

 parts of North America, and in both respects more species and more 

 individuals. In part this is no doubt due to the fact that "over 

 there " a large part of the land has been deforested and cultivated 

 for centuries, and the snails had all that time to accommodate them- 

 selves to changed conditions. However, it must be taken into 

 account that most of western and middle Europe has a more or less 

 maritime climate, with higher humidity of the air and less severe 

 changes of temperature, than most of North America, which is more 

 conlinental. At such places of the Old World, as mentioned, there 

 are the many species of various groups of Helix (s. lat.), TorquiUa 

 and other Pupidce, Clausilia, etc., most of which are out in daylight, 

 while our PoJygyrince and most other land snails (few of them con- 

 spicuous) are much more retiring and not, or rarely, seen during 

 the day. 



One effect of civilization and international and intercontinental 

 trade and travel is the introduction of mollusca from one country or 



