STROBILOPSIDAE 



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STROBILOPS AFFINIS Pilsbry 



The shell of Strobilops affinis differs from that of Strobilops 

 labyrinthica in being larger. The glossy brown shell of affinis 

 measures nearly one-eighth inch (2.50-2.75 mm.) in diameter. 

 Shells of the two species differ also 

 in that affinis has a wider umbilicus 

 and coarser ribs on the surface. In 

 affinis only the large parietal lamella 

 emerges from the shell, whereas in 

 labyrinthica two lamellae are visible 

 at the aperture. The shell of affinis 

 is more nearly conical in form than 

 that of labyrinthica. 



The basal folds of affinis are 8 

 in number, 5 of which are distinctly 

 visible through the translucent base 

 of the shell. These basal folds are 

 much shorter than are those in laby- 

 rinthica and form a radial series to- 

 ward the aperture. The parietal la- 

 mellae penetrate more deeply than do 

 those of labyrinthica labyrinthica but 

 not so far as in the form that is known as labyrinthica parietalis. 



This comparatively large Strobilops is known only from 

 northeastern Illinois in the counties of Lake, McHenry, Kane 

 and Cook. It is a forest snail found in moist woodlands under 

 loose bark and on old decaying wood. 



A few years ago little was known concerning the species 

 of the genus Strobilops, but thanks to the work of Dr. H. A. 

 Pilsbry, Curator of Mollusca in the Academy of Natural 

 Sciences of Philadelphia, the group has been carefully studied 

 and a number of new species and races have been discovered. 

 Of the nine species and varieties found in the United States 

 and Canada, seven are known from Illinois. Recognition of 

 specific variations has come about as a result of the method Dr. 

 Pilsbry used of opening the shell for the examination of the 

 lamellae and folds. The family Strobilopsidae has been well 

 discussed and the species beautifully illustrated in Volume 28 

 of the Manual of Conchology, published by the Academy of 

 Natural Sciences of Philadelphia in 1927. 



