NO. 2.] SNAILS OF THE GENUS 10— ADAMS. 53 



point strongly to the probability that this fauna is native and developed in place with the 

 evolution of this part of the continent and its drainage. Such considerations, combined with 

 others derived from the distribution of lo in the different parts of the Tennessee system, are 

 favorable to the view that lo has always been solely confined to the draiaage now included in 

 the Tennessee system. lo hterally "grew up with the country." 



DEVELOPMENT OF THE SHELL AND ITS SCULPTURE. 



1. INTRODUCTORY. 



The development of the environment and the geographic origin of the family Pleuroceridse 

 have been discussed. We turn now to the development or "orderly sequence" of the changes 

 in the shell and its sculpture. This, in contrast with the quantitative descriptive data, is a 

 descriptive historical study of the development of the shell. This is based upon the compara- 

 tive study of immature shells, the adult stages of which are, of course, unkno^vn, and also upon 

 the study of the older whorls of adult shells, where the intermediate stages are generally fairly 

 well preserved, but whose older apical whorls are frequently destroyed. These records are often 

 fragmentary, but this defect is overcome in part \>y the large number of individuals studied, 

 and by a knowledge of the probable Mnes of development, which comes with a knowledge of the 

 general relations within the genus as a whole. Thus, for example, all of the immature shells 

 found in the Nolichuchy are spinose and, therefore, it is considered very probable that all are, 

 even though the apical whorls of some may be eroded and the spines destroyed upon these whorls. 



It is remarkable that, in spite of the extensive collections made, relatively so few very young 

 shells were found, although particular attention was given to them; and further, that large 

 numbers of immature shells were found in relatively few locahties. This suggests that there 

 may be a lack of uniformity in the breeding season, as it hardly seems possible that the young 

 were overlooked in so many locahties. These animals must be relatively sedentary, and it 

 seems probable that the young frequent the same habitat as the adults. 



In order to secure a just basis for comparison, it has been necessary to devise some estimate 

 of the age of individuals. Size is perhaps the most rehable criterion; but after the first season's 

 growth, erosion of the apex may be so marked that measurements of the total length of the 

 shell are useless for determining age. Then again, size is greatly influenced by the dimensions 

 of the stream in which the shells occur; small shells occur in small streams and large shells in 

 large streams. For this reason, there is no absolute test of size which can be applied. Its value 

 is, therefore, relative. 



The number of whorls is a valuable index in the case of young and perfect shells; but the 

 frequent destruction of the older whorls by erosion limits the general application of such a test. 

 The diameter of the shell, as measured on the last whorl, is one of the most reliable tests of 

 age, but this is also limited by the fact that some shells, which are below the normal in size for 

 a given locality, show distinct signs of old age. The relative thinness of the shell is character- 

 istic of immaturity, and this is shown by the thin or sharp-edged peristome and the feeble devel- 

 opment or lack of a varix or scar which marks the position where growth is resumed after a 

 period of rest. This resting period, in general, we may assume to cover the winter season, 

 although it is probable that several kinds of stimuli may produce the same result. 



Maturity and old age may thus generally be recognized by the large size, or large whorls 

 (when the apex is defective) , the thickness or heaviness of the shell, a thick peristome, and the 

 distinct varix, which marks the position of a period of arrested growth. Old age may further 

 be indicated by the worn apex or truncated eroded apex and the extreme heaviness or thickness 

 of the shell. The increased number of varices upon a single whorl also appears to be an index 

 of advancing age, showing that the rate of growth has diminished with age. 



It is thus seen that there are several lines of evidence by means of which age may be esti- 

 mated, and in practice it is seldom desirable to depend upon any single test. 



In order to have some basis for comparisons, the shells have been roughly sorted into five 

 classes, as foUows: 



Class 1. This includes the youngest shells found. These have been considered shells of 

 the first season's growth, as they were collected in the fall. There is considerable variation 



