1915.] NATURAL SCIENCES OF PHILADELPHIA. 409 



Sonorella sitiens arida n. subsp. PI. VIII, figs. 6, 6ct, 66. 



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 1| whorls show distinct spirally 

 protractive threads in young individuals. 



Alt. 10.8, diam. 19 mm.; aperture, alt. 9, cUam. 10.2 mm.; umbihcus 

 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.; 

 umbihcus 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. S. 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 

 <;rag 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 mdth 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. tuma- 



