OP PLANORBIS AT STEINHEIM. 67 



the young of Steinheimensis and the adult of parvus and the very evident differences 

 between the full grown shells of each variety. They are both probably distinct varieties 

 of PI. levis derived directly from that species. Some of the specimens of one of the 

 varieties of PI. levis from Undorf are identical with parvus and have been previously 

 described.-' 



Hilgendorf regarded ^arwMS as intermediate hetweeia. Tninutus and var. aequiumiilicatus, 

 and also as in the same genetic series as crescens. Sandberger on the other hand joined 

 minutus, teres, and crescens under the name of Zieteni Braun as a distinct species. 



Flanorbis crescens. 



PI. m. crescens Hilg., Op. cit., fig. 16. 



This species is perhaps the least variable of any of the Steinheim forms. The 

 connection with the preceding is clearly made through some specimens slightly stouter 

 than the norm, but these are exceedingly rare in my collection. 



The mouth and last whorl may be central or turned downwards. I have so far 

 seen none with these parts deflected upwards or against the spiral. Some of those 

 with the mouth in the middle have nearly equal umbUici, but these are extremely 

 rare forms. Those with the mouth turned downwards and the upper umbilicus only 

 slightly marked, as in figs. 9—12, line c, pi. 3, and those with no upper umbilicus, as- in 

 figs. 13, 14, on the same line, are very numerous. Quite a trochiform variety ends 

 the tendency to variation in this direction, of which I have found one specimen, 

 pi. 9, fig. 16, but even in this one the whorls retain the same attenuated aspect and 

 form. The third carina is prominent in all of these, and in some the fourth makes 

 its appearance, especially in those like figs. 13, 14, line e, pi. 3. The striae of growth 

 are particularly well defined in this variety, even at the earliest stages, and in aU 

 specimens. 



Remarks. On pi. 5, the entire genetic series as here described, is figured. The four 

 shells on line a are undoubtedly PI. levis, from Undorf, and show the close relationship 

 with PI. parvus from the Pits, as exhibited on line h. PI. "parm! is exhibited on line c 

 and on line d the ordinary forms of PI. crescens. 



The angularity of the outer whorls resulting from the development of the third 

 carination is evidently a mark of affinity with PL levis, in which this is a constant 

 characteristic. The gradual fiattening of the form of the whorl is shown in the right 

 and left series of figures, and the close resemblance in form of the whorl to that 

 of PI. levis, in fig. 2, line b. Compare this with some of the forms of PI. levis, on 

 pi. 7, which are in the same position. This flattening of the angularity of the whorl, 

 and the openness of the lower umbihcus. fig. 5, line d, are both low characteristics, 

 and show that the series has altered but very little in the characteristics which 

 were derived from PI. levis. The increase in size is notable, and this must be 

 classed with the progressive series. The progression in the flattening of the whorls 

 and the angularity of the outer whorl which takes place in the adult also characterizes 

 the young, and is inherited at earlier stages in each form, until, in fig. 2, the young 

 begin to shbw the crescens form at a very early age. 



1 See p. 35. 



