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. 



ZONITID^. 

 Vitrea indentata umbilicata (Ckll.). 



Two miles west of Bisbee, and about the same distance east of 

 Warren, on limestone hills, with Holospira. 



PUPILLIDJE. 

 Bilidaria 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 

 El 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 debris 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 debris near the S. P. R. R. 

 bridge. In this sample the most abundant moUusk is Bifidaria 

 procera cristata. The small Zonitoides, Bifidaria p. hordeacella, 

 Pupoides marginata and Vertigo ovata are next in abundance. AU 

 the species except Vallonia gracilicosta are Lower Sonoran forms. 



Thysanophora hornii (Gabb). 



Holospira ferrissi Pils. (variety). See p. 388. 



'' F. FOSSOR P. and F. See p. 387. 



" n. sp. (spire only). 



Vitrea indentata umbilicata (Ckll.). 

 Zonitoides minuscula alachuana (Dall). 



" SINGLEYANA (Pils). 



