The Variation and Distribution, Recent and Fossil, of the Snail Polygyra profunda Say, in Illinois

Abstract

Polygyra profunda is a land snail that occurs abundantly in the northern portion of the United States and is one of the species most commonly encountered in the northern section of Illinois. lt is known from early Pleistocene, during which period it seems to have been abundant. The purpose of the present investigation is to ascertain the variation within the species, both recent and fossil, its distribution within the State both at present and during the Pleistocene period, and its range in geological time. The material for the investigation consists of several hundreds of specimens from Illinois colletced by Daniels, Hinkley, Nason and by members of the State Natural History Survey, as well as of many lots from regions outside the State. The fossil material consists principally of collections made by field parties of the Illinois State Geological Survey. The nature of the variation in size and other features was determined by the biometric method and the measurements are plotted on graphs (Figs. 216) to determine the mean of variation-the mode. In this, actual measurements have been used rather than averages. By this means the largest and smallest variations may be compared for each lot and subject. As measure-