1915.] NATURAL SCIENCES OF PHILADELPHIA. 409 
Sonorella sitiens arida n. subsp. PI. VIII, figs. 6, 6a, 6b. 
The shell resembles S. sitiens, but differs in these features: the 
umbilicus is decidedly wider, its diameter contained 6 to nearly 7 
times in that of the shell; the color is paler; the aperture is notice- 
ably smaller. The embryonic 14 whorls show distinct spirally 
protractive threads in young individuals. 
Alt. 10.8, diam. 19 mm.; aperture, alt. 9, diam. 10.2 mm.; umbilicus 
3 mm. 
Alt. 10, diam. 18.5 mm.; aperture, alt. 9, diam. 10 mm.; umbilicus 
3 mm. 
Alt. 10.25, diam. 19.9 mm.; aperture, alt. 9.9, diam. 11. mm.; 
umbilicus 2.9 mm. 
Cerro Colorado, around the base of a conspicuous crag at the south- 
eastern end of the range. Types No. 112,160, A. N. 8. P., collected 
by Pilsbry and Ferriss, September 28, 1910. 
The first two measurements are of cotypes from the south side of 
the crag. The third specimen measured is the only adult shell taken 
on the north side of the crag, perhaps a hundred feet higher. 
This form stands very close to S. sitiens, yet the difference in the 
size of umbilicus is constant in the small series examined; no com- 
munication between the colonies of Cerro Colorado and Las Gijas 
can have taken place for a very long period, so that in the present 
state of our knowledge it seems proper to keep the forms of the two 
hill-groups subspecifically separate. 
Unfortunately, no living examples were found, so that the ana- 
tomical characterization of the subspecies remains to be worked out. 
The Cerro Colorado (‘‘Red Hill’’) lies a few hours’ travel north of 
Las Gijas. The northern slopes are grassy and rounded, but west 
and south it is carved into bold, fantastic crags and pinnacles of 
dull red rhyolite—a landmark which catches the eye for a long distance. 
Our work here was brief. Scarcely an hour was spent around a 
crag which stands at the southeastern extremity, about two miles 
from the Cerro Colorado Mine on the Aravaca Road. Here the 
Sonorella described above was taken, only a few dead specimens. 
No doubt, the cliffs westward, higher up, would yield better results, 
though little can be expected in such a dry situation. Neighboring 
low crags of milk-white quartz, at a lower level southward, were 
found barren. 
Sonorella sitiens comobabiensis n. subsp. 
The shell is similar to S. sitiens in general shape, its width contained 
about 9 times in the greatest diameter of the shell. It is smaller 
within, and enlarges more in the last whorl than that of S. twma- 
