1915.} NATURAL SCIENCES OF PHILADELPHIA. 389 
floated many miles, as the river merits that name in time of flood, 
though usually reduced to a chain of infrequent pools or an insig- 
nificant rivulet. The term river, in the arid belt, refers to the bed 
and banks rather than to the water, which is often conspicuous for 
its absence during a great part of the year. 
ZONITIDA. 
Vitrea indentata umbilicata (CkIl.). 
Two miles west of Bisbee, and about the same distance east of 
Warren, on limestone hills, with Holospira. 
PUPILLID. 
Bitidaria pellucida hordeacella (Pils.). 
Limestone hill about 2 miles east of Warren, Arizona. 
III. Benson, ARIZONA. 
Benson, Cochise Co., at the junction of the Southern Pacific and 
Ei Paso and Southwestern Railroads, is in a flat region, with no mol- 
lusk fauna in its immediate environs. The San Pedro River, flowing 
northward about a mile east of the town, brings down considerable 
flood débris containing shells. The source of these is probably in the 
foothills of the Whetstone Mountains, not far away; possibly also 
the hill country about Tombstone, or even further south. 
The San Pedro carries more water than any other stream in the 
lower tier of counties between the Rio Grande and the Colorado, 
and so far as we know it is the only one maintaining a constant flow. 
At Benson it is a turbid stream 20 to 30 feet wide, with vertical, 
dirt banks about 8 feet high (September 1st), meandering in a flood 
plain covered with mesquite. 
Mr. Ferriss collected a few shells from the river drift in 1904; and 
in 1910 Pilsbry and Daniels, having an hour or two between trains, 
collected a small bag of shell-bearing débris near the 8S. P. R. R. 
bridge. In this sample the most abundant mollusk is bifidaria 
procera cristata. The small Zonitoides, Bifidaria p. hordeacella, 
Pupoides marginata and Vertigo ovata are next in abundance. All 
the species except Vallonia gracilicosta are Lower Sonoran forms. 
THYSANOPHORA HORNII (Gabb). 
Ho.LosPIRA FERRISSI Pils. (variety). See p. 388. 
i F. FossoR P. and F. See p. 387. 
te n. sp. (spire only). 
VITREA INDENTATA UMBILICATA (CkIl.). 
ZONITOIDES MINUSCULA ALACHUANA (Dall). 
SINGLEYANA (Pils). 
