76 PROCEEDINGS OF THE ACADEMY OF [Feb., 



noticeably expanded, basal margin somewhat more so; columellar 

 margin rather broadly dilated and brought forward. The parietal 

 callus is short, thin and transparent. 



Alt. 7.2, diam. 14.5 mm.; aperture 7.5 mm. wide, 7 high. Whorls 



4i 



Distribution: North side of White Tail Canyon, in slides of igneous 

 rock (rhyolite), at Stations 10, 11, 15, 16, 17. Type locality, Station 10. 

 See map on p. 75. 



This is the smallest Sonorella yet known from the Chiricahuas. It 

 is allied to S. virilis by the genitalia and the indistinct sculpture 

 pattern on the embryonic whorls, but differs by its diminutive size, 

 thin shell, comparatively narrower umbilicus and the absence of 

 distinct white borders along the brown band. 



S. micra occupies the north side of White Tail Canyon to the exclu- 

 sion of S. virilis leucura, which lives on the opposite or south side. 

 It lives in "slides" of angular, dark purplish-gray rock (rhyolite), 

 together with Ashmunella lepiderma, and can be found only deep in 

 the rocks, where they rest on the earth below. The slopes are mainly 

 toward the south and west, hence exposed to the direct sun, rarely 

 shaded to any extent by the small trees. The snails are found from 

 the bottom of the canyon (Station 11) to perhaps 6,800 feet (Station 

 16), where they occur over the crest of the ridge, in an extensive slide 

 sloping northward. The type locality, Station 10, is below the great 

 cliffs along Indian Creek. The lower stations are easily accessible, 

 but the higher call for arduous climbing and, from the nature of their 

 haunts, the work of quarrying the snails out is severe. We secured 

 but few living specimens, but many dead shells, more or less fresh, 

 show the species to be remarkably constant. Besides a slight varia- 

 tion in elevation of the spire, no variation in form is noticeable. 

 Measurements of two lots follow : 



Diam.inmm 12.75 13 13.25 13.5 13.75 14 14.25 



Station 10 1 12 7 4 



Station 17 10 4 4 10 4 



Diam.inmm 14.5 14.75 15 15.25 



Station 10 4 7 7 1 



Station 17 13 2 



On the southern side of the canyon we found a few dead specimens 

 among limestone rocks about 20 feet above the bed of the canyon, 

 below the junction of Indian Creek. They^agree fully with those 

 from the north side of the canyon. 



