1911.] NATURAL SCIENCES OF PHILADELPHIA. 139 



looking, and this may account for the apparent stunting shown towards 

 the end of their shells after the reproductive organs have developed, 

 as indicated in the curves for "whorl 5" and "width." 

 Pleurodonte acuta goniasmos A. D. B. 



This very variable species is the common large Pleurodonte in the 

 Mandeville district. Some 500 specimens of adult shells were col- 

 lected in this region from the definite colonies above described, as 

 well as about a hundred immature shells; and, in addition, about 

 150 others from mixed colonies were taken, which are not considered 

 in this comparative study. The largest collection was from the 

 Somerset colonies, a suite of 229 being secured here of the adult 

 shells, as well as a large number of young. Measurements of all of 

 these six hundred shells (adult and immature) from definite colonies 

 were made, and from these curves showing the distribution of dimen- 

 sions plotted as well as the whole series of dimensions. From the 

 young and from breaking down the whorls of adult and young shells 

 measurements were secured in each case, from which measurements 

 the growth curves of the development of the shells were worked out. 

 Besides these measurements of height and width (the two dimensions 

 recorded for all specimens), in the case of each colony, a small number 

 of typical shells were selected and measured as to the diameter at 

 each whorl, and these were averaged (as well as the heights and widths, 

 and the indices with the mean divergence) and all these factors were 

 plotted to scale. From this last diagram (fig. 6) it is at once seen 

 that the differences in dimensions vary with the geographical position 

 of the colonies and that at least two lines of migration are involved 

 in this region. In the tract examined these begin at Somerset and at 

 the Kendal Road colony, respectively, and, making two series of the 

 colonies arranged in geographical order from these points, two sets 

 of curves are at once obtained in which the graduation in dimensions 

 is regular from one end to the other (fig. 6). The two lines, the 

 Somerset wave and the Kendal Road wave, converge from the north- 

 west and from the northeast towards Mandeville, where they are only 

 separated by about a half-mile; and, doubtless, to the south of Mande- 

 ville colonies could be found that show a mingling of these two races. 

 That they were originally one there is no doubt, but they branched 

 off to the north and east from the main common stock and, following 

 the topography of the country and the most favorable ground, became 

 separated, but eventually came together again. In this migration 

 through the "Manchester backwoods," as Chitty calls this region, 

 the animals undoubtedly followed the lines of least resistance, where 



