ZONITIDAE 77 



Genus STRIATURA Morse 



Five species and races of Striatura are found in the United 

 States. The one species and one race occurring in Illinois are 

 characterized by their small size and by the surface sculpture 

 of raised folds or ribs. 



STRIATURA MILIUM (Morse) 



One of the smallest species of land snails found in the state, 

 Striatura milium has a greenish-white shell measuring less than 

 one-sixteenth inch (1.5 mm.) in diameter. It may be recognized 

 by its diminutive form, its widely 

 open umbilicus and the sculpture 

 of the 3 whorls, which consists of 

 many raised folds or ribs that at 

 times run together. In the upper 

 surface of these folds, fine im- 

 pressed lines cross the vertical ribs 

 in such a manner as to form a reticulated surface sculpture. 



Striatura milium milium (Morse). In the typical variety, 

 shown in the figure, the apical or nuclear whorls are smooth 

 or only faintly lined. The spiral lines on the lower whorl are 

 faintly developed. 



This variety lives in woodlands in old decaying wood. The 

 only Illinois locality from which it has been recorded is in 

 McHenry County. 



Striatura milium meridionalis (Pilsbry & Ferriss). The 

 variety Striatura milium meridionalis, found in the southern 

 part of Illinois, differs from the northern form in certain con- 

 stant variations. It is a trifle larger. Strong, incised spiral 

 lines mark the apical or nuclear whorl. The spiral lines are 

 distinct in the base of the shell and are conspicuous in the widely 

 open umbilicus. The oblique wrinkles or ribs are also much 

 coarser in meridionalis than in the typical shell. This sculpture 

 can be seen only with the aid of a good hand lens magnifying 

 12 to 15 diameters, or with a microscope. 



The form meridionalis, noted in Washington and Monroe 

 counties, appears to be the dominant variety of milium in 

 southern Illinois. It is probably an example of a migrant from 

 the south, since meridionalis is common there. 



