PUPILLIDAE 



107 



VERTIGO MILIUM (Gould) 



Smallest of Illinois vertigos, milium has a shell that meas- 

 ures about one-sixteenth inch (1.50-1.75 mm.) in height. In 

 color the shell is dark amber or chestnut. It has 4 to 5 rounded 

 whorls, and its general shape is globose. 

 The aperture has 6 folds and lamellae: a 

 parietal, an angular and a columellar la- 

 mella, a basal fold, and an upper and a lower 

 palatal fold. The palatal folds differ from 

 those of other species found in Illinois. They 

 are very long and extend far backward into 

 the aperture. The small size of the shell, 

 the shape of the palatal folds and the down- 

 ward ending of the columellar lamella dis- 

 tinguish the species Vertigo milium from all 

 other pupoids. 



This minute pupoid is well distributed over Illinois, but the 

 records are scattered. Its small size has doubtless caused it to 

 be overlooked. Its habitat is under sticks and debris in moist 

 floodplain areas and in woods on bluffs bordering the large 

 rivers of the state. 



J'ertigo milium is so peculiar in its apertural folds and 

 lamellae that Dr. Victor Sterki placed it in a subgenus which 

 he christened Angustula. In this group the columellar lamella 

 is crescentic in shape and its inner end curves downward, instead 

 of upward, as in other species of Vertigo. Only two species are 

 known to belong to this subgenus: milium, the distribution of 

 which is from Maine to Florida, west to South Dakota and Colo- 

 rado and south to Arizona, Tampico, Mexico, and Jamaica; 

 and bermudensis Pilsbry, which is found only on the Island of 

 Bermuda. 



It is noteworthy that a species having such a wide geographi- 

 cal distribution should not show some varietal changes some- 

 where in its range, but this species is remarkably uniform in its 

 general characteristics wherever found. The consistency of 

 milium over a large area leads to a consideration of one of the 

 mysteries of animal life. Why one species should be uniform 

 over a wide territory and another develop a number of races 

 or varieties, as is the case in several species of Gastrocopta, is at 

 present concealed from human knowledge. 



