114 INTRODUCTION. 



is that they are more numerous in humid localities 

 than elsewhere. For the same reason, they prefer the 

 bottom of ravines, and the damp and shady recesses of 

 the forest, and choose for their places of shelter strata 

 of wet and decaying leaves, and the lower surfaces of 

 wood and stones in contact with the soil, or even bury 

 themselves in the soil. For though they have the ability 

 to retire into their shells and of hermetically closing the 

 aperture, yet this power seems to be provided as a de- 

 fence against occasional or accidental changes, and not 

 against constant or long-continued eifects. Hence, they 

 are rarely found to occupy positions that are perma- 

 nently arid ; and in regions subject to long-continued 

 drought, and upon soils which easily give up their mois- 

 ture, they are not met with except ia low situations, 

 near the margins of streams, ponds and swamps. For 

 the same reason they are not common on soils which 

 become hard and impenetrable on drying, and are want- 

 ing in sun-burnt wastes and in sandy deserts. The vicin- 

 ity of waterfalls, within the influence of the spray and 

 mist arising from them, is a favorite locality of many 

 species. Excessive moisture, on the other hand, is very 

 prejudicial, for they cannot exist in swamps, or upon 

 grounds subject to be overflowed with water, and they 

 are destroyed by inundations. The genera, however, and 

 even the species, diSbr considerably in these partic- 

 ulars. Glandina occupies habitually wet and swampy 

 grounds, and Sacdnea and some species of Pupa are 

 often fomid upon ground in the immediate vicinity of, 



