OF PLANORBIS AT STEINHEIM. 69 



Variety typica. 



The second variety, or true norm, which I take to be identical with Hilgendorf s 

 figure, has the mouth and part of the last whorl considerably contracted, the lower 

 umbilicus quite deep, and the lower sides of the whorls gibbous but rounded, figs. 12-18, 

 line k, and fig. 12, line &, pi. 3. 



The third variety, figs. 2-9, line m, jjl. 3, has a mouth similar to the first variety in 

 some specimens, but in others there is a lip of medium thickness on the inner side.^ This 

 form is an exaggerated repetition of that of figs. 12-16, line a, j^l. 1. The closeness of the 

 resemblance in form is due to the rotundity of the lower side, and the depth of and 

 narrowness of the mnbilici in both forms. 



Planorbis supremus. 



PI. m. supremus Hilg., Op. cit., fig. 9. 

 First variety. 



This begins with a form precisely similar to the preceding, except in the upper umbil- 

 cus. This deepens by growth, and the first carina and sulcation begin to make their 

 appearance. The lower side also, is sharper, and the fourth carina stands out quite promi- 

 nently. The mouth is central. Figs. 6, 10-12, line o, figs. 7-9 same line, are inter- 

 mediate between these and the last described form of oxysfomtis on line k, pi. 3. 

 Second variety. 



This differs only in having the mouth bent downwards. The first carina and 

 sulcation become very prominent, and the fourth carina also, as in figs. 1-7, line n, 

 and 1, 5, line^, pi. 3. 



Another form is represented by figs. 8-13, line n, and fig. 1, line o, pi. 3. In these 

 the upper umbilicus becomes shallower, and in fact almost disappears. This is one of the 

 nearest approaches to a turretted form observed in this variety, and it is accompanied in 

 fig. 8, line n, by the development of a distinct sulcation on the upper side. A typical 

 variety of oxystomus occurs on line a, fig. 14, pi. 3, in formation h, East Pit. This diflers 

 from the series just described in the upper umbilicus. This is shallower, and the inner 

 whorls are therefore more exposed in the young. Almost without exception, these 

 have the fourth carina strongly developed, but there are some in which the upper umbilicus 

 is deeper than usual, and the whorls more cylindrical. 



This variety leads by the closest gradation into the stouter form figured on line h, pi. 3, 

 figs. 1-6, and line I, figs. 1-3. The next step is shown in figs. 9-10, line h, and 5-8, Ime c, 

 in which the lower umbilicus may be either quite wide or very narrow, and the upper side 

 begins to lose the umbilical depression. 



In fig. 13, line h, fig. 1, line c, and figs. 4-7, line I, pi. 3, this tendency is consummated 

 in a truly turretted form. 

 Variety turrita. 



In figs. 9-11, line Z, pi. 3, the whorls increase with great rapidity below the fourth carina, 

 and narrow lower umbilicus, assuming a trochiform aspect. 



I have one specimen of this last, considerably larger then the one figured, with a form 

 similar to that of " elegans " Hilgendorf It is noticable that although the first and fourth 



1 It should be noticed here that the specimen figured on by a very thin or ahnost imperceptible film, while in PL 



line a, pi. 1, fig. 12, has a thickened inner lip, also a char- oxystomus the opposite is the case, the thin film being excep- 



acteristic which is exceedingly rare in PL Steinheimemis, tional, and the thick deposit the rule. 

 in which this portion of the mouth is usually represented 



