Transformations of Planorbis. [June, 



and the four first forms of each series, viz: 2 and 9 and 

 occur in the Steinheim basin, is very complete, but 

 ly left out in this plate. 



Numbering the series from right to left we see that Series 1 has 

 three sub-series. Two of these show a tendency to uncoil, to 

 become distorted and smaller than the ancestor, Fig. 16, while the 

 third decreases in sizi, but has a form, Fig. 20, which is turreted 

 like Figs. 1 1 and 6. 



Series n maintains a size about the same throughout, but be- 

 comes flatter than the originating form, Fig. 12. Series III grows 

 sensibly larger, and 10-n are turreted-shells with a more rigid 

 and sub-angular form of whorl than the primal form of Fig. 8. 



Series iv exhibits not only greater increase in size, but vastly 

 greater differences in form and in other characteristics of the shell 

 from Fig, I, with which it started. 



We can, therefore, without fear of error call series iv a highly 

 . progressive series ; Series ir a psrsistent series; sub-series 3 of 

 Series 1 a partly retrogressive series ; sub-series 2 of Series 1, a 

 purely retrogressive, and sub-series I of Series I also partly retro- 

 gressive ; since, though it decreases in size and becomes deformed 

 and .uncoiled, it also has a tendency to produce a new character- 

 istic, the transverse ribs, and also increases in size its more closely 

 coiled forms, as in Fig. 26. 



There is also other testimony going to show that this classifi- 

 cation is correct. Semper's researches on Lymnceus stagnalis show, 

 that under the most favorable physical conditions, this species 

 increases to a maximum of size and has larger whorls, while 

 under less favorable conditions with relation to food and tempera- 

 ture, the size is very much decreased. 



The immediate results of weakness, produced by wounds, are 

 also important in this connection. PI. vir, Figs. 21, 2\a and 22, a 

 •diseased PL oxystomus, var. reve 

 of the same species as Fig. 9, 

 partly uncoiled, shell with the species, Fig 23, PL VI, PL denu- 

 datus and minutus. The weakness consequent upon old age is 

 equally significant and has a similar meaning. PI. vn, Fig- 22, 

 represents the effect of old age in distorting the growth o( the 

 outer whorl of PL oxystomus. Compare with Fig. 22, PI. VI - 

 These are true cases of disease of comparatively rare occur- 

 rence in PL oxystomus. I have in my collection many similar 



