16 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 487 



Like many other species of Gyraulus , altissimus is characterized by 

 Zh to 4 rapidly increasing whorls and the lack of a distinctive umbilicus. It is 

 recognized by the fact that the periphery of the shell is situated in a ventral 

 position; the ventral whorl is not flattened as in G. parvus. G. altissimus is the 

 only species of the genus recognized in these faunal assemblages (fig. 3), 

 where it occurs in great numbers at every level studied, but many shells tend 

 toward G. arotious and some tend to approach parvus. The dense populations of 

 G. altissimus in all faunal assemblages make this species a significant element 

 of the molluscan faunas in these bog lakes. 



Genus Armiger Hartman 1840 



There seems little to justify this genus as distinct from Gyraulus except 

 the shell sculpture; the soft anatomy is similar to that of species of Gyraulus. 

 Granting validity to the genus, species of Armiger are found in North America, 

 Europe, and Asia. 



Armiger exigua Leonard 1972 



Armiger exigua Leonard 1972, Nautilus, v. 85, p. 81, figs. 1-2 and 1-3. 



This tiny gastropod closely resembles A. crista, but is much smaller. 

 It has the same number of whorls; the nuclear whorl is granular rather than 

 striate; there is no indication of spiral striation; and the last whorl does not 

 descend toward the aperture, which is roundly oval in shape. It occurs sparingly 

 in five of the faunal assemblages (fig. 3), none of them in the Batavia locality 

 (136). Nothing is known, of course, of the ecological requirements of this mi- 

 nute gastropod beyond that which can be deduced from its associates. The 

 species was described from shells found in Petersburg Silt of early Illinoian age, 

 Henry County, Illinois. It has been found in late Woodfordian sediments in 

 eastern New Mexico. 



Genus Helisoma Swain son 1840 



Species of Helisoma are among the large gastropods of the family 

 Planorbidae. Shells are robust, the cross striation is typically coarse, and 

 spiral striation may be conspicuous. 



Helisoma antrosa (Conrad) 1834 



Planorbis antrosus Conrad 1834, Amer. Jour. Sci., 1st ser., v. 25, p. 343 

 Helisoma antrosa (Conrad), F. C. Baker 1928, Freshwater Moll. 

 Wisconsin, pt. 1, p. 317, pi. 19, figs. 8-15. 



Helisoma antrosa is another of the six species that occur in every faunal 

 assemblage in the localities studied (fig. 3) although population numbers are 

 lower than those of any other of the persistent species except Physa gyrina 

 hildrethiana. Nevertheless, this species forms a conspicuous element of the 

 total molluscan assemblages. 



Many, but not all, of the examples collected have the spiral striations 

 that characterize a described form {striata F. C. Baker) that supposedly 

 characterizes Pleistocene populations of H. antrosa. It seems unlikely that the 

 "striata" modification of sculpture has taxonomic significance. 



The aperture of many Helisoma antrosa specimens is distended in a bell- 

 like form resembling H. campanulatum , but the spires of the two species are 

 entirely different; that of H. antrosa forms a deep, smooth-sided funnel, while 



