46 PROCEEDINGS OF THE ACADEMY OF [Feb., 



can fauna as full as circumstances permitted. In many of the canyons 

 we have endeavored to locate the individual snail colonies with suffi- 

 cient detail to insure their recognition by subsequent observers, so 

 that their further evolution may be followed. Large areas still 

 remain to be explored, and neither author has had time to fully study 

 the material collected. 



The first record of mollusks from the Chiricahuas was made by 

 Dr. R. E. C. Stearns, who in 1890 describes specimens of Holospira 

 arizonensis, 3 collected by Mr. Vernon Bailey for the U. S. Department 

 of Agriculturejat Dos Cabezas, in the western foothills of the Dos 

 Cabezas Mountains. In 1895 Dr. W. H. Dall described Polygyra 

 chiricahuana and subsequently (1897) he reported Pyramidula striatella, 

 Thysanophora ingersolli and Zonitoides arborea,* all from Fly's Park 

 in the central Chiricahuas. collected by Dr. Fisher. No other species 

 were known from the range prior to the first visit by Mr. Ferriss in 

 February, 1904. Numerous new species were found during this brief 

 visit, notwithstanding the unfavorable season. In November, 1906, 

 both of us collected in the range, exploring the principal canyons from 

 Buckeye in the north to Cave Creek and the parks about its head. 

 In November, 1907, Messrs. Ferriss and L. E. Daniels spent two 

 weeks in the Chiricahuas, and in 1908, from September 20 to November 

 15, Ferriss continued the work of exploring the southern canyons. 



I. Conditions Determining the Isolation of Snail Colonies. 



The faunas of the several mountain ranges of southern Arizona 

 are separated one from another by the intervening nearly level mesa, 

 where snails are absolutely wanting and cannot exist. This is due not 

 alone to their greater aridity, higher temperature and xerophytic 

 flora, but chiefly we believe to the absence of rocks, in the interstices 

 of which snails might burrow below the dry surface to depths where 

 a certain amount of moisture is retained. The mesa forms a barrier 

 as impassable to land snails as an equal expanse of sea; and can be 

 surmounted only by minute forms light enough to be transported by 

 the wind. During the existence of the present conditions, w r hich 

 probably were initiated in the Pliocene, the larger snails of each range 

 have been absolutely isolated. 



Owing to the general north and south trend of the ranges, the main 

 canyons run eastward or westward, thus exposing a very hot slope 



3 Proc. U. S. Nat. Museum, XIII, 1890. 



4 Proc. U. S. Nat. Museum, XVIII, 1895, and XIX, 1897. 



