8 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 304 



Finally, the composition of successive populations is profoundly influenced 

 by the relative genetic stability of the organisms under consideration. The Mollusca 

 seem to have long since passed the peak of their adaptive radiation and, when com- 

 pared with many other kinds of animals, to have reached a high degree of genetic 

 stability. Although there is ample evidence that terrestrial and fresh-water mollusks 

 are still making evolutionary progress, their genetic conservatism and the brief 

 interval of time involved suggest that faunal changes within the Wisconsinan result 

 from migrations in response to fluctuating ecological conditions. 



These considerations lead to the conclusion that assemblages of fossil mol- 

 lusks at any stratigraphic level may have distinctive character over a considerable 

 area if the ecological conditions are similar and relatively stable. The species- 

 composition of a fauna may comprise well known taxa, found perhaps in younger 

 and in older rocks. Distinctive qualities of a specific assemblage need not include 

 extinctions, the presence of newly evolved taxa, or even so-called "index- fossils"; 

 as we have pointed out, nearly identical assemblages may reappear from time to 

 time in the stratigraphic column without nullifying their significance and usefulness 

 at any specific stratigraphic level. All molluscan fossils considered in this report 

 are housed in the paleontological collections of the Illinois State Geological Survey. 



Altonian Faunas 



Altonian molluscan faunal assemblages are shown in figure 2, in which lo- 

 calities of occurrence are arranged geographically, and a summary of Altonian oc- 

 currences appears on figure 3, in which relationships between Altonian and other 

 Wisconsinan faunal assemblages are shown. 



Inasmuch as Altonian faunas have been recovered only from the Roxana silt, 

 it is not surprising that the northernmost locality of occurrence should be in extreme 

 southeastern Schuyler County (locality 29). Ten additional localities are distributed 

 southward to Alexander County (locality 58b). As the Roxana silt generally thickens 

 southward, there is a corresponding increase in the variety of species as well as in 

 density of the local populations in the southern localities. This effect can be attri- 

 buted to at least two factors: improved preservation of shells in the thicker deposits 

 and probably more favorable ecological conditions in the south, where the forests 

 were denser and more stable. 



A total of 28 species of mollusks are known to occur in the Roxana silt, al- 

 though the average local fauna comprises no more than half this number. Among 

 these, six species, Angulsplra alternnta, Haplotrema concavum , Rendersonia occulta, 

 Stenotrema fraternum, S. hlrsutum, and Succinea ovalis, are found in all three of the 

 Wisconsinan substages — Altonian, Farmdalian, and Woodfordian. Eighteen species, 

 including Clone lla lubrica, Discus cronkhitei, D. maccllntorkl , Gastrocopta arml- 

 fera, G. contractu , Hawaiia mlnuscula. , Hellcodlscus pornllelus, Mesodon thyroldus , 

 Pomatiopsis scalarls, Punctum mlnutlsslmum, Retlnella electrlna, Strobllops laby- 

 rlnthlca, Succinea gelida, S. grosvenori, Vallonla graclllcosta, V. parvula, Vertigo 

 hubrlchtl (not shown on fig. 2), and Zonitoldes arboreus occur in Altonian and Wood- 

 fordian deposits but have not been found in sediments of Farmdalian age. Two spe- 

 cies, Allogona profunda and Aniulsplra kochi, range from Altonian into Farmdalian 

 deposits, but have not been recovered from Woodfordian sediments. Only two spe- 

 cies, Trlodopsis hubrlchtl and T. altonensls, seem clearly restricted to the Altonian. 

 Allogona profunda also is restricted to the Altonian except for its occurrence in the 

 Farmdalian at the Shawneetown locality (57b) . 



