OF PLANORBIS AT STEINHEIM. 83 



the direct line, except in the case of those of PI. troch'iformis, which have the discoidean 

 young and are descended from those just referred to, as figured on Une m, pi. 2. Thus the 

 forms in the direct line of descent have the advantage in point of size, as well as in the 

 other characteristics mentioned. 



Besides these numerous varieties, there are still others which cannot strictly perhaps 

 be attributed to disease or any definable cause. One of these is precisely similar to fig. 4, 

 line r, plate 2, in the rotundatus-like characteristics of the apex, but is extremely attenuated 

 or tapering. The whorls, however, are perfectly regular, and the external aspect of the 

 spiral smooth and more regular even than in fig. 4. This seems to be the variety 

 vermetiformis Sand. 



Another variation, which may be the trochiformis communis Sand., is that represented 

 by the peculiar stoutrwhorled spiral of figs. 4-9, Une Tc ; figs. 2-5 and 6-8, line s, on 

 plate 2, have also rotundatus-like young. 



The distorted specimens, line r, figs. 9-10, plate 2, also deserve special attention. These 

 have rotundatus-like young, but the last whorl, as shown in fig. 10, has an excessively 

 thick inner lip, and strikes off from the regular line of increment. In both cases, the 

 coarseness and the crumpled look of the aberrant j^art of the whorl indicates a diseased 

 condition to which doubtless the distortion must be attributed. 



There are also extraordinary forms frequently found at the apex, showing that the 

 young, as in fig. 1, line s, plate 2, must have presented a remarkable spiral during the 

 two whorls, owing to the sharp projection of the second carina due to the contraction of 

 the spiral. This was undoubtedly, I think, a pathological condition, because if it had 

 been continued the adult would have been much distorted, perhaps even unrolled, as in 

 PI. denudatus Hilg. It will be perceived that these results agree only in part with those 

 of Hilgendorf. That author traces PI. trochiformis through discoideus and suleatus in 

 linear succession, to PI. tenuis, then to PL Steinheimensis, and lastly to PL aequiumbil- 

 icatus. 



The changes or variations from sidcatus and discoideus varieties into trochiformis were 

 probably simultaneous or nearly so. At any rate, though this cannot be proved, there is 

 a strong antecedent probability in its favor, owing to the sudden appearance of these 

 forms in such great numbers, the precise parallelism of the different series, the frequency 

 of hybrids and a parallelism so exact that it can only be explained by supposing that it 

 was produced by the intercrossing of all the varieties. 



This conclusion I find after a re-examination of the material to be substantially correct 

 so far as the evidence as found in the Pits is concerned. • 



Throughout the preceding observations any reader can see by referring to the figures 

 from what Pit and formation the different shells came, but in the following remarks no 

 attention is paid to the formation, that not being considered important. 



There are upon plate 7, six specimens of PL leois from Undorf ; three of them are 

 easily picked out, having been mounted on white paper, but can any reader distinguish 

 the other three from the Pit forms of PL ^'^"tvlT'"'^^ j which are associated with them, before 

 reading the following descriptions. 



This series is arranged entirely with the view of showing the zoological afl&nities of the 

 series, independent of their geological relations. 



