MONTANA SHELLS. 261 



will the second person fall to find some overlooked specimens, so 

 closely do they harmonize with their surroundings. This blending of 

 colors applies with almost equal force to the species living in water. 

 Markings may refer to spots or bands on the shell, or to the indentations 

 on the surface. This latter may be coarse or fine, deep or shallow, 

 numerous or few. It may be possible to count the number per inch or 

 millmeter, and thus determine points of variation. 



The Spiral. This may be right or left handed, dextrose or levulose. 

 Most shells are dextrose. If shells are in quantity each should be ex- 

 amined carefully to see if perchance an accidental specimen may be found 

 the reverse of the ordinary. Such cases occur. Some species have the 

 shells left handed, turning opposite to the hands of a watch. Each 

 should be examined. The number of turns in the spiral should be 

 counted. A means should be devised for determining the fractional turn 

 at the last, as it is very likely to result in a fraction. By making count 

 of a series variations will be found and a mean established. 



Diameter and Depth. Adult specimens should be used. Three 

 straight edges are necessary, one of which should be a finely graduated 

 ruler. Place the shell against the ruler, and put the two remaining 

 straight edges on either side, also against the ruler. The reading may 

 be taken from the ruler directly. With a ruler and two square blocks 

 any number of shells may be quickly measured. By turning the speci- 

 men measurements may be made in different ways, and variations noted. 

 It is understood that the value of such work to the pupil depends largely 

 on whether it is done for him or by him. 



Variations. In a mountainous region shells of a given species from 

 different localities or altitudes will show many variations. In fact, not 

 two shells from any place are exactly alike. By noting the above points 

 many lines of variation may be noted. It is possible to determine the 

 direction toward which the species is tending; i. e., it is becoming thicker 

 shelled, with deeper markings, broader bands, fewer turns to the spire, 

 less width and depth, smaller in body, or the reverse. All such observa- 

 tions afford food for reflection, and are excellent mental stimuli. As 

 this is the main thing sought in nature study work the observations 

 should be encouraged by each individual, and not by the class as a whole. 



Food. Few suggestions need be given on this. In the snailery dif- 

 ferent things must be tried. Daily observations may show whether land 

 forms eat living or dead leaves, decaying wood, wet or dry leaves; whether 

 water snails eat living plants or decaying material in the water, or living 

 animals. All observations should be carefully recorded, and at stated 

 times notes may be compared. 



Enemies and Unfavorable Conditions. Drouth kills most land snails. 

 Those living in water have no doubt many enemies about which little can 

 be determined. Permit some shells to become quite dry and watch their 

 actions. Notice the film across the shell to prevent evaporation. Other 

 enemies to land snails are rodents, including mice and squirrels. If 

 broken shells are found, examine carefully to see if the break is by acci- 

 dent after death, as by washing among rocks, or by an enemy before 

 death. This must be determined by the position and character of the 



