78 



ZONITIDAE 



Genus ZONITOIDES Lehmann 



The shells of Zonitoides, about the size of those of Retinella 

 and Paravitrea, do not have the latter's tightly coiled whorls, 

 nor the former's spaced, impressed lines on the body whorl 



which extend from the suture to the base. All species of Zoni- 

 toides have depressed, shining shells, strikingly umbilicated. The 

 lines of growth are visible to the human eye only when aided 

 by a simple lens. Fine microscopic spiral lines are also present in 

 some species, but a powerful magnifying glass is needed to find 

 them. The figure shows the shell and animal of Zonitoides 

 arboreus, which is one of the most common of the land snails 

 found in the state of Illinois. 



Key to Species 



1. Umbilicus wide and shallow; surface of shell sculptured. . . . 



Ii7jjatnlus, p. 80 



Umbilicus deep and narrow, p. 79; surface of shell smooth 

 and shining 2 



2. Aperture round, p. 80; umbilical opening one-fifth diameter 



of shell; diameter of shell about ^4 inch (6.0-7.5 mm.) .... 



nJtiduSy p. 79 



Aperture ovate, p. 79; umbilical opening one-sixth 

 diameter of shell; diameter of shell less than l<i inch 

 (5.0 mm.) arboreta, p. 78 



ZONITOIDES ARBOREUS (Say) 



The snail Zonitoides arboreus is smaller than Zonitoides 

 nitidus, but is in many other respects similar to it. The yellow- 



