PUPILLA, AMERICAN. 



171 



two in Colorado only single shells were taken. In a few 

 colonies, so far as our information goes, the dextral form 



FIG. 3. P. s. dextroversa P. & V. Bass Trail, Grand Canyon, about 200 ft. 

 below the rim. Lengths 4.5 and 3.76 mm. 



occurs as a pure race, and many more colonies are of purely 

 sinistral shells. 



The Colorado records quoted from Cockerell and Hender- 

 son are widely detached from those in New Mexico and Ari- 

 zona. All of the material from the latter states has been ex- 

 amined by the writer. 



P. syngenes avus Pils. & Ferr. Fig. 4. 



Shell sinistral, the last whorl deviating tangentially and 

 ascending; teeth deeply immersed; parietal lamella much 

 longer than in P. syngen-es or d-extroversa, about a half -whorl 

 long. 



Length 5.2, diam. 1.8 mm., whorls 



Length 4.3, diam. 1.7 mm., whorls 



Length 4.0, diam. 1.7 mm., whorls 



Types no. 94,220 A. N. S. P., from upper slope of the Grand 

 Canyon along the Mystic Spring or Bass Trail, about 200 feet 

 below the rim; abundant with P. s. dextroversa. 



The special characters of this race, being those of senility, 

 are unequally developed in different individuals. The figures 

 give a fair idea of the variations. Finding these shells asso- 



