1911.] NATURAL SCIENCES OF PHILADELPHIA. 187 



comment on the several lots follows, beginning at the northern 

 stations. 8 



(1) Jacob's Canyon. (PI. XIII, figs. 1-5), Oreohelices were taken 

 at Stations 68, 69. 70. 71. The typical form and coloring (fig. 1) 

 prevail, but there is also one color-form not elsewhere encountered, 

 in which the bands are purple-brown and very wide, the upper one 

 spreading to the suture or leaving a white belt below the latter (figs. 

 2-4). There are also some pearl-white and dirty white shells (fig. 5). 

 The same color-forms occurred at Station 67, below the mouth of 



6 Only one of us (Ferriss) collected P. S. depressa north of the Grand Canyon, 

 being accompanied there by Mr. L. E. Daniels. His impressions concerning the 

 relations of shell-characters to the environmental factors of elevation, humidity 

 and direction of slope are given below. It must be remembered that the con- 

 ditions in the Kaibab region are less accentuated than in the more arid moun- 

 tains of the south. 



"At the time our observations in 'Mollusca of the Southwestern States, 

 No. IV,' were written, environment seemed the controlling factor in the deter- 

 mination of size of the shell, northern exposures, with an abundance of shade 

 and plant life and a longer growing season, would produce. the larger shells. 

 In the region north of the Grand ( anyon many apparent exceptions to this 

 rule were noted. Often colonies of the same species but 100 feet apart varied 

 100 per cent, in size. In a gulch facing north in the Powell-Kaibab saddle, in a 

 box canyon with perfect snail conditions, the Oreohelix average 20 mm. in diam- 

 eter. Above the box, in a more open country, they average 24 mm. and were 

 more plentiful. 



"In the canyons and amphitheaters of these plateaux, with the same exposure, 

 moisture, shade, elevation — mineral, plant and all other conditions equal so far 

 as we could understand — each colony stood out by itself in color, size and shape 

 of the spire. These qualities seemed subject to mutation rather than con- 

 trolled by environment. 



"Thus, in Two-Spring Canyon, the shells in a colony on one side of a rock 

 were 2.5 per cent, larger than those of the colony upon the other side, less than 

 100 feet distant ; and a colony on the west side of the stream, no farther away, 

 was larger than either. In the center of a colony on Powell Plateau, a 'family' 

 of larger shells was found. 



"Until we crossed the Kaibab Plateau, the collections of 1909 were at about 

 the same elevation, in the limestone section. In Two-Spring and Quaking Asp 

 Canyons, both heavily wooded, the Oreohelices at the top, among the quaking 

 asp, were 14 mm. in diameter, gradually increasing in size, as we descended, 

 to 25 mm., as would naturally be expected. 



"In Snake Gulch, however, the largest shells were at the top, on slopes facing 

 either north, west or south, the diameters running 25 and 26 mm. at Castle 

 Springs and vicinity. At Big Springs, with abundance of shade and humidity 

 the year around, and a warm southern exposure, the largest measurements were 

 21, and at the lowest colony hi the gulch less than IS mm. 



"In Warm Springs Canyon, running from east to west parallel with Snake 

 Gulch, the smaller colonies were midway in the canyon, those at both the top 

 and bottom of the canyon being unusually large and robust, and it was the same 

 in Jacob's Canyon, another parallel canyon of this group running to the west. 



"We collected Oreohelix at 22 stations, elevation about 6,700 feet, in the saddle 

 region of the Powell and Kaibab Plateaux; at 19 stations in Two-Spring and 

 Quaking Asp Canyons in the Kaibab Plateau, at elevations between 6,500 and 

 8,250 ; at 20 stations in Snake Gulch, Kaibab Plateau, from 5,000 to 7,000 feet 

 elevation ; at 16 stations in Warm Spring and Jacob's Canyons, Kaibab Plateau,, 

 from 6,000 to 6,750 feet elevation." 



