1906.) NATURAL SCIENCES OF PHILADELPHIA. 155 
their intervals, and about 21 in number in a millimeter measured 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 mm. 
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). 
‘'\ == 
—> 
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. The 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 considered 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: 
Es 
a.—Diam. of shell 3 to 3.5 mm., whorls 4 to 44; teeth almost always 
present. Eastern United States, . . - H. parallelus (Say). 
a'.—Diam. of shell 4.5 to 5 mm., whorls 4} to 53; often deficient in 
teeth. 
b.—Spiral strie obsolete. Idaho. . . H. salmonensis (Hemph.). 
_ eo.  — 
