1906.] NATURAL SCIENCES OF PHILADELPHIA. 155 



their intervals, and about 21 in number in a millimeter measm-ed at the 

 periphery, in the front of the last whorl. The last whorl is rounded 

 throughout. The umbilicus is widely open, its width contained about 

 three times in the diameter of the shell. The aperture is slightly 

 oblique and deeply lunate. Alt. 1.1, diam. 2 mm.; diam. of umbilicus 

 .6 to .7 nam. 



Huachuca mountains, Cochise county, Arizona, (J. H. Ferriss) ; Carr 

 canyon (Dr. Henry Skinner). Also in the State of Michoacan, Mexico, 

 at Patzcuaro and Morelia (S. N. Rhoads, 1899). 



Fig. 10. 



This tiny Patuloid, of a group new to our fauna, is one of the 

 most interesting recent finds. It is related to R. hermanni (Helix 

 hermanni Pfr.), of central and eastern Mexico, but that species is 

 larger and more coarsely sculptured. Thfe two species occur together 

 at the Mexican localities mentioned above, but are readily separable. 

 Dr. Henry Skinner found one broken specimen of R. millecostatus in 

 Carr canyon, Huachucas. 



Genus HELICODISCUS Morse. 



This genus has hitherto been consitlered to include two species, 

 lineatus (Say) and fimbriatus Wetherby. The accessions of Southwest- 

 ern material show that several other forms must be recognized. The 

 species are not very conspicuously differentiated, and young shells by 

 themselves are not always readily placed ; yet with adults we find no 

 difficulty. The salient characters of the forms follow: 



a. — Diam. of shell 3 to 3.5 mm., whorls 4 to 4^; teeth almost always 



present. Eastern United States, . . . H. parallelus (Say). 



a^. — Diam. of shell 4.5 to 5 mm., whorls 4^ to 5+; often deficient in 



teeth. 



b. — Spiral striae obsolete. Idaho. . . H. salmonensis (Hemph.). 



