406 PROCEEDINGS OF THE ACADEMY OF [Juil? 



Alt. 12.7, diam. 21.3 mm.; whorls 4|. 

 " 9.9, " 17 

 " 9, " 16 " " 41. 



There was a scalariform specimen among the bones. It measures 

 13.3 mm. high, 16.6 wide. The normal height for a shell of this 

 diameter should be about 9.5 mm. 



The Mineral Hill group. Twin Buttes and Tinaja Hills are much 

 degraded outliers of the Sierrita Mountains. Only the Mineral 

 Hill group has been worked for land snails, though all doubtless 

 have Sonorellas — and very little else. 



The Mineral Hills are about 20 miles west of south from Tucson 

 and about 7 miles north of the Sierritas.^ They stand at the summit 

 of a long slope, rising about 1,000 feet in ten miles from San Xavier 

 del Bac, on a mesa of perhaps 3,600 feet elevation. The xerophytic 

 vegetation extends over the hills, mesquite, cat-claw, palo verde, 

 ocotillo and sotol being the more conspicuous plants, to which may 

 be added tree cacti on southern slopes, and on the mesa many opun- 

 tias, cylindropuntias and a few barrel cacti and yuccas. The 

 absence of Agave is peculiar. These hills are a favorite resort of 

 rattlesnakes. I got also a coral snake. No mollusks whatever 

 were found on Mineral Hill or Helmet Peak. San Xavier Hill is 

 composed of white subcarboniferous limestone, like the hills south- 

 eastward, except at the western end, which is whitish quartz, with a 

 spur to the north of coarse pinkish-gray granite. There is a depres- 

 sion in this end of the hill, between short, low cliffs of white quartz. 

 The cliff towards the south has partly fallen in a tumble of huge 

 blocks with some smaller stone between them. This talus is perhaps 

 200 feet long to the last scattered blocks, and at the widest 40 feet 

 wide; its lower end about 200 feet above the mesa. In it we found 

 the Sonorella described above. "Bones" were abundant, but living 

 snails extremely scarce, and confined to the deeper portions of the 

 talus, between the piled-up quartz blocks. The entire range of this 

 species is not much greater than the area occupied by a moderate- 

 sized house. In this insignificant fastness it is making a last stand 

 against extermination. 



We found no snails in a hill covered with granite boulders about 

 3 miles north of west from San Xavier Hill. It is possible, though 

 unlikely, that some insignificant colony may exist there. 



s While there we occupied a comfortable camp at the copper mine of Mr. 

 L. D. Chilson, of Tucson, whose courtesy we would here acknowledge. 



