•54 PROCEEDINGS OF THE ACADEMY OF [Feb., 



form a strong argument against modification by the action of environ- 

 ment. The differentiation affects internal organs concerned solely 

 in the act of reproduction, and only in the most remote manner con- 

 nected with parts directly acted upon by external factors. 



Genus SONORELLA Pilsbry. 



Sonorella is the most characteristic Helicid snail of the mountains 

 of southern New Mexico and Arizona. It is more widely distributed 

 than either Ashmunella or Oreohelix, inhabiting mountains too arid to 

 support either of the other genera, such as the Peloncillo, Dos Cabezas 

 and Rincon ranges. In the Grand Canyon of the Colorado we dug 

 them from the cruelly sharp rock-slides of the pre-Cambrian zone, in 

 deep, sunless canyon- where no other snails penetrate; and they exist 

 equally on the plateau at the rim, 5,000 feet above the sea. 



Tins tolerance of untoward conditions is doubtless due in part to their 

 deep digging habit. Sonorella is usually found well down in the 

 rocky talus or slide, in the layer against the earth, where a certain 

 degree of moisture remains. In some places Ashmunella accompanies 

 Sonorella in the same slides. This Is especially the case in the more 

 verdant and humid canyons, such as Cave Creek in the Chiricahuas, 

 where Sonorella, Ashmunella and Oreohelix clappi may be found 

 together. 



Oreohelix (except 0. clappi and 0. barbata) lives on the surface, on 

 or under the top stones, or around dead mescal, bear grass or yuccas. 

 While apparently fitted to live in exposed and extreme^ arid places, 

 yet in southern Arizona Oreohelix certainly does not inhabit the more 

 arid ranges where Sonorella exists. 



Oreohelix and Ashmunella may sometimes be collected in copious 

 quantity, but Sonorella is almost always rare. In the Dos Cabezas 

 range a half dozen living adults would be a good day's bag for any 

 collector, and in dry weather, at least, this would mean very hard 

 work. In le^s arid localities the snails are less scarce; but as a general 

 rule to collect living Sonorellas successfully is the most strenuous 

 physical labor, occasionally enlivened by danger of broken limbs from 

 falling rocks or caving in of holes, in slides of heavy rock. Strong 

 gloves are necessary in " quarrying" for Sonorella. In spite of strained 

 muscles, bruises and bleeding fingers, the fascination of Sonorella 

 hunting is irresistible. The game gains in value from the hardihood 

 brought out in the chase, and it is only after one has forgotten the 

 labor that he can part with a single specimen of the rarer species. 



Chiricahuan Sonorellas belong to two very diverse groups distin- 

 guished as follows : 



