20) FRIENDS WORTH KNOWING. 
able, late in the summer, to rake ont the bottom of mud- 
holes where the water has entirely disappeared. Another | 
plan is gently to pull up the water-weeds by the roots, and 
cleanse them in a basin of water. You will thus secure 
many very small species. Experience will quickly teach 
the collector where he may expect to find this and that 
kind, and that some caution and much sharpness of obser- 
vation are necessary, since some species by their naturally 
dead tints, and others by a coating of mud, assimilate them- 
selves so nearly to their surroundings as easily to be over- 
looked by man as well as other enemies. 
The shell is increased rapidly for the first two or three 
years, and the delicate lines of increment, parallel with the 
outlines of the aperture, are readily visible on all the larger 
specimens. Various other signs indicate youth or adult 
age in the shell. 
Mollusks prosper best, ceteris paribus, in a broken land- 
scape, with plenty of lime in the soil. The reason, no doubt, 
why the West India Islands, the Cumberland Mountains, 
and similar regions are so peculiarly rich in shells of every 
sort, is that a ravine-cut surface and a wide area of lime- 
stone rocks. characterize those districts; on the other hand, 
it is not surprising that I found nine-tenths of the Rocky 
Mountain species to be minute, since the geology is repre- 
