176 PROCEEDINGS OF THE ACADEMY OF [March, 



Powell Survey, long. 113°, lat. 36° 27', on a limestone ridge between 

 Finley's reservoir and Mt. Trumbull. Station 42, ant-hills on same 

 road westward. 



North of the northern rim of the Grand Canyon only one species 

 of large size was found, — Oreohelix strigosa depressa (Ckll.); but this 

 occurs in a multitude of colonies and in great variety of size and color. 

 So far as we know, this form does not occur in Arizona south of the 

 Colorado River in the Grand Canyon. The small shells are for the 

 most part generally distributed on both sides of and in the Grand 

 Canyon. Special to the Grand Canyon, so far as we know, are the 

 following forms : 



Sonorella Colorado ensis (Stearns). 



Oreohelix yavapai, subspecies extremitatis , angelica and profundorum 

 P. and F. 



Pupilla syngenes avast P. and F. 



Only the first of these is generally distributed, being found on 

 both sides of the river, and for at least 30 miles along its course; 

 also on the plateau south of the canyon at Bass Station. 



The work of collecting in the Grand Canyon is severe. The trails, 

 except the Bright Angel or tourists' trail, are narrow and very steep. 

 You will dig snails on taluses ending in cliffs dropping hundreds of 

 feet ; no sound comes back from the rocks your work dislodges. Also, 

 the sandstone and metamorphic rock is hard on the hands. Oreohelix, in 

 restricted localities, is sometimes very abundant, but Sonorella is 

 always rare, and living ones can be obtained only by hard work. 

 Satisfactory work along the south side can de done from the camps or 

 hotels at the several trails. For work in the lower levels and on the 

 north side of the river a camp outfit and pack animals are required. 

 These may be obtained at the Bright Angel, Bass, Grand View or 

 Hance Trails. 



The most productive horizons in the Grand Canyon are the Kaibab 

 Limestone, which forms the slope immediately below the rim, and 

 the talus at the foot of the Cross-bed or Coconino Sandstone, in shel- 

 tered recesses where a talus from the overlying limestone terrain has 

 accumulated. The deeper levels are comparatively unproductive, 

 though Sonorella penetrates in suitable places nearly to the river. 



The molluscan fauna of the Grand Canyon, with the sole exception 

 of Sonorella, consists of species inhabiting northern Arizona on one or 

 both sides of the canyon or of forms evidently derived from such species- 

 It must, therefore, so far as mollusks are concerned, be considered a 

 part of the Transition zone. Sonorella is the sole Upper Austral genus. 

 It inhabits both sides of the river, up to and even upon the rim. 



