o4 PROCEEDINGS OF THE ACADEMY OF (Feb., 
form astrong 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 canyons where no other snails penetrate; and they exist 
equally on the plateau at the rim, 5,000 feet above the sea. 
This 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 O. clappi and O. 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 extremely 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 less 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 ts 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: 
