1910.] NATURAL SCIENCES OF PHILADELPHIA. 73 



and they probably never persist in the adult stage. All immature 

 specimens taken by us in November, from diam. 10 mm. on, have 

 a strong callous rib within the lip-edge. 



Adult stage. — There are 4^ whorls in small, 4f in large individuals 

 of the type lot, slowly increasing at first, the last whorl seen from above 

 about twice as wide as the penultimate. The lip is blunt with rusty edge, 

 but not thickened within. The outer margin is slightly expanded, the 

 basal a trifle more so; parietal callus rather thick at the edge, straight. 

 The umbilicus contained about 5.2 times in the total diameter. Alt. 

 11.8, diam. 23.5 mm.; aperture 10 x 11.3 mm. 



Distribution: Southern side of White Tail Canyon, Chiricahua 

 Mountains, Stations 1, 2, 3, 4, 5, 9, 12, 14. Type locality, Station 

 No. 14. Also in Jhu Canyon. See map, fig. 6, p. 75. 



This is the only Sonorella on the southern side of White Tail Canyon, 

 living on steep slopes varying from northeast to north or northwest. 

 It occurs (in November) rather deep under rocks, and excepting on 

 the more arid slopes and ridges, colonies may be found where suitable 

 shelter occurs throughout the length of the canyon, generally rather 

 high on the slopes, but in at least one case only about 20 feet above 

 the bed of the canyon. The more favorable slopes, such as those 

 Stations 5, 14, 15 are situated on, are wooded with pinyon below, 

 oak and a few long-leaf pine above. The ridges and slopes with 

 southern or eastern exposure are barren or nearly so. The rock is 

 limestone, mostly with covering of earth and fine stone, and there are 

 no rock slides. Stations 1, 2, 4, and 9 are decidedly more arid and 

 barren than 5 and 14, Station 5 being near the bottom of a deep narrow 

 ravine, densely wooded, while Station 14 is on a steep shady northern 

 slope. The lots taken show that the shells respond to more favorable 

 conditions by reaching a larger size, as seen in the table of measurements. 



The shells from Station 5 have the spire somewhat more depressed 

 than any other lot. Those from near Jim Artels' old camp, near head 

 of White Tail not far from Station 1 , are the smallest. 



The table following shows the shells from high on the ridge (Stations 

 1 and 4) to be in the main smaller than those from the mid-slope ; but 

 this ridge becomes very dry above, with less shade, while the slope 

 lower down is well shaded on northwest exposures (caused by the 

 lateral washes and ravines, running northward), the northeast exposures 

 being for the most part barren. The size of individuals in the colonies 

 collected appear, therefore, to be related to the amount of shade and 

 moisture. 



