OF PLANORBIS AT STEINHEIM. 85 



carina, sulcation and narrow umbilicus make their appearance, figs. 5-10, line e, pi. 1. 

 The square form of the whorl as inherited by PI. discoideus passes through a similar 

 series of descendants, in which it becomes more pronounced and stouter in the young by 

 degrees, imtil the young themselves from being similar to tenuis become stout whorled and 

 discoidean, as well as the adults, fig. 16, line b, pi. 2, fig. 14, line g, pi. 1. The turbinated 

 form of the shell is wholly an adult characteristic in many specimens of discoideus, figs. 

 2-3, line h, pi. 1, fig. 12, line p, and may be traced with aU its accompanying characteristics 

 through several series to PL trochiformis, where the different series culminate in shells 

 which have young that are so trochiform-like at an early age, that only the very youngest 

 periods retain any resemblance to discoideus. 



In some of these it is easy also by breaking down the shell to see one of the common 

 results of this mode of inheritance by acceleration. Namely, the Steinheimensis form is 

 almost, and in some few cases entirely, unrecognizable. It is skipped by the development, 

 and so also is the tenuis form, the discoideus form alone surviving in the young.-' - 



"With regard to the proper identification of these forms, and those described by Klein in 

 Jahresh. d. Vereins fiir vaterland. Naturk. in Wiirtt., 1847, p. 60, fig. 7, line h, pi. 1, 

 seems to me identical with Klein's V., multiformis var. planorbiformis, pi. 2, fig. 14. 



Fig. 7, line k, pi. 1, seems to correspond with Klein's V. multiform,is var. intermedia, 

 pi. 2, fig. 15. Fig. 8, line r, pi. 2, is almost exactly identical except in size with Klein's 

 figure of V. m,ultiform,is var. trochiformis, pi. 2, fig. 16. Fig. 7, line I, pi. 2, is equally 

 close to Klein's figure of V. m,ultiformis var. turbiniformis, pi. 2, fig. 17. Fig. 11, line o, 

 pi. 2, is apparently identical in every respect with Klein's V. multiformis var. rotundata, 

 pi. 2, fig. 18. 



Feeling that it would be futile to attempt a revision of the names of the different 

 varieties, I do not attempt here or elsewhere, to do anything more than assist those who 

 may feel disposed to take upon themselves this thankless task, with such observations as I 

 may have made. 



Sandberger's diagnosis, in which he divides this group into three, is the best which has 

 been devised, with the exception of roiundatus, which, as shown, is only a name for a 

 young specimen, and is therefore placed in brackets. 



First variety^ A, planorbiformis; containing sub- variety a, sulcatus, sub- variety b, 

 discoideus, sub-variety ^, elatior. 



Second variety B, intermedius ; containing sub-variety a, communis, sub-variety b, 

 elegans, [sub- variety c, rotundatus'\. 



Third variety C, trochiformis ; containing sub-variety a, communis, sub-variety b, 

 pyrguliformis, sub-variety c, vermetiformis. 



I This phenomenon was what led to the adoption of the ^In place of "variety," I should have written group, and 



name " acceleration." ^n place of " sub-variety " the word species in most cases, 



but this is merely a difference in taste. 



