OF PLANORBIS AT STEINHEIM. 77 



The majority of this variety, however, have the square form of the whorl with the 

 sulcations and carime well developed. The mouth in these may be deflected against the 

 spiral slightly, as in fig. 16, line c, pi. 1, and perfectly flat on the upper side, as in figs. 15, 

 17, 18, on same line. They are inseparable from the preceding variety, though in many 

 forms they tend to grow in a sub-turretted form, fig. 8, line d, pi. 1, and line e, figs. 14, 

 15, and line 0, fig. 1-3, fig. 6, and line/, pi. 7. 



These last lead without break into forms such as figs. 4-7, line 0, which have 

 an extraordinary development of the first carina with a sub-turretted shell, or with a flat 

 shell, as in figs. 2-10, line e, pi. 1. Then a sub-variety, probably connected with the last, 

 in which both the first and fourth carinas are very prominent, as in figs. 11-13, line e, 

 pi. 1. • 



The forms on line n, pi. 1, figs. 9-12, are much compressed, with sides flattened and 

 convergent outwardly, showing a whorl, which connects them with the extremely 

 flattened form, fig. 8, same line ; and also fig. 1, of line h, on same plate. 



If one examines this last mentioned compressed form, and fig. 8, line n, there is in 

 both a slight want of symmetry, which consists in the greater prominence of the zone 

 which would be occupied by the third carma, if it were present. 



This is perhaps one of the most curious of the genetic series traced out directly from 

 specimens, which are identical with Steinheimensis, on account of the almost imperceptible 

 changes of form by which it is accomplished, and by reason of the extreme variation of 

 some of the varieties. 



Varietv '''^'^"'^^"^ 



Planorbis discoideus. 



! 



tenuis ' 



This transition form, when that term is used in its most conservative sense, as applicable 

 to shells which exhibit characteristics which make it impossible to decide whether they 

 belong to discoideus or to tenuis, is not found in the Pit Deposits. It is, however, 

 found in the Cloister Ridge rocks and has been previously described. 



These forms entirely fill the gap, since they meet the only objection which can 

 be urged against the intermediate position of the Pit forms ; they have young which 

 " precisely resemble the adult of PL tenuis." There is indeed so close an ajiproximation 

 to true transition forms that I have more than once had to alter the nomenclature of this 

 series. Thus the gap which exists between the form of PI. tenuis figured on line e, fig. 1, 

 pi. 1, and those of PI. discoideus figured online/, figs. 4, 17, pi. 1, is so slight that hardly 

 any naturalist would hesitate to unite the former with the latter. Nevertheless the young 

 of the form figured on line/ cannot be considered as similar to the adult of the form 

 figured on line e, since it is stouter.' 



For a precisely similar reason I have also been obliged to separate the compressed 

 forms of discoideus figured on line h, figs. 10-12, and line p. figs. 1-2, pi. 1, from PI. 

 tenuis, though among these it is possible that farther search would detect PI. 



discoideus 

 tenuis * 



' This would generally be considered as of no value, but it adopted is restricted to tbe most conservative basis of anti- 

 must bo remembered that the basis of the reasonino; here evolutionism. 



