POLYGYRIDAE 53 



POLYGYRA OBSTRICTA (Say) 



The brown shell of Polygyra obstricta, about an inch in diam- 

 eter (20-22 mm.), differs from that of Polygyra palliata in hav- 

 ing a flatter spire and a carina which is sharp and keellike at the 

 periphery, as though pinched 

 with the fingers. The surface 

 of obstricta, unlike that of palli- 

 ata, is not covered with hairlike 

 processes of the epidermis. The 

 denticulation in the aperture is 

 about the same in both species. 



Animals of these species are similar in color, size and shape. 



Polygyra obstricta is probably the rarest species of land snail 

 in Illinois and is known only from the valley of Big Creek, in 

 Clark County, a tributary of the Wabash River. The ravine 

 in which it was collected is forested with oak, elm, hickory and 

 some sycamore, and the ground is plentifully supplied with old 

 logs and other forest debris. Polygyra obstricta is also rare in 

 Indiana, its nearest locality in that state being Posey County. 



Some years ago the late L. E. Daniels, a careful conchologi- 

 cal student of Indiana MoUusca, collected a series of Polygyra 

 obstricta which shows a remarkable relationship between ob- 

 stricta and palliata, two species usually markedly distinct. These 

 were submitted to Dr. Henry A. Pilsbry who commented on 

 them as follows: "A series of 18 specimens from Grand Chain, 

 Posey County, taken by Mr. Daniels in bottom land near the 

 Wabash River, and in its floodplain shows a complete series of 

 transitions from palliata to obstricta. These supposed species 

 have been separated not only on account of the acute keel of 

 obstricta, and its absence in palliata, but more because of the 

 rough epidermis of the latter. This series, as far as I can see, 

 shows intergradation in the development of the cuticular proc- 

 esses, as well as in the contour of the shell, 



"A few of the specimens agree almost exactly with Say's 

 type specimen of palliata, the periphery being moderately angu- 

 lar, the angle disappearing on the last third or fourth of the 

 whorl; and the surface, besides having low, coarse striae, bears 

 numerous cuticular asperities, as though a loose cuticle had been 

 pinched up into many little points, and more or less wrinkled 

 in consequence between them.' 



