POLYGYRA. 71 



but by way of the Bahama bank, which had previously been 

 connected with Cuba and Haiti. 



The question of the relationships of Polygyra is beset with diffi- 

 culties. I had formerly grouped the genus with Pyramidula, etc, 

 but the characters of the foot peremptorily forbid such association, 

 Dr. v. Ihering suggests the possibility that it may be either a modi- 

 fied branch of Arionta in which thegenitalia have become simple by 

 degeneration, or a further development of Patula. The latter hypoth- 

 esis is untenable. The former has as yet no facts to support it. 



No fossils now known throw light upon the problem. From what 

 we know of the living forms of Polygyra, it is likely that their 

 common ancestor possessed a shell with tridentate aperture, reflected 

 lip, and a color-band above the periphery. It is not unlikely that 

 the group represents an early stage of the true Helix phylum, which 

 did not share the evolution of the accessory organs of the genitalia 

 now characteristic of the Pentatcenia, Campy Icea, Cochlostyla, etc. 



Polygyra divides into three sections, typically very distinct in 

 in appearance, but closely connected by more or less intermediate 

 species. The anatomy is practically the same throughout. 



Section Polygyra Say, (restricted). 



Shell depressed; umbilicated, or having a curved groove caused 

 by the tangential deviation of the last whorl. Aperture somewhat 

 kidney-shaped or ear-shaped, the lip continued in an elevated v-shaped 

 callus across the parietal wall ; outer lip having two teeth or none. 

 Type P. septemvolva Say, pi. 30, figs. 1, 2, 3. (See also pi. 30, fig. 

 4, P. auriculata Say). 



Central teeth tricuspid, the side cusps well developed ; laterals 

 bicuspid ; marginal teeth generally having the mesocone bifid at tip, 

 at least on the extreme margin of the radula, ectocone simple (pi. 

 30, fig. 5, P. septemvolva ; pi. 30, fig. 7, postelliana'). Genital system 

 as described above (pi. 30, fig. 6, P. troostiana). 



This section comprises some very aberrant species, but the 

 extremes are so closely connected by intermediate forms that no use- 

 ful subdivisions can be maintained. The synonymy of the restricted 

 section Polygyra comprises the names Dtxdalocheila, Ulostoma and 

 Cyclodoma. 



The species inhabit the Southern States, a few ranging as far north 

 as South Carolina, Kentucky and Missouri, extending southward 

 throughout Mexico. In the West Indies species are found in the 



