OF PLANORBIS AT STEINHEIM. 81 



look to the true discoideiis line for the origin of the major portion of the trochiform 

 varieties. That this is the case, will be established by looking at any large collection 

 of specimens.^ 



In the majority of the varieties of PI. trochiformis, the upper sides of the whorls 

 of the young are exposed at the apex of the shell, and these are smooth, not sulcated, 

 and have two carinae generally blunt, as in the true discoideus. 



Others, however, equal to the variety pyr.guliformis Sand., PI. trochif. turhinatus 

 Hilg., are more prominently carinated, as on plate 2, figs. 9-12, line s, and have flattened 

 apices ; and the young, as seen externally at the apex, seem to be identical with 

 the young of such forms as are figured on line n, figs. 1-2, plate 2, which are flattened 

 forms of discoideus, distingdishable from the true sulcatus variation by the absence of a 

 deep sulcation on the upper side. 



Occasionally, however, the apices of some true trochiform varieties are similar to those 

 of the young of other varieties than discoideus. Thus, in the case previously pointed out, 

 the young of the forms on line r, figs. 6-7, plate 2, indicates a descent from the variety 

 elatior, fig. 16, line i, same plate, and to this might be added other similar forms like 

 fig. 8, also on line r, plate 2, and also fig. 8, line Jc, plate 1. These appear to be identical 

 with Klein's V. multiformis, var. trochiformis, and in part with Sandberger's multiformis, 

 var. trochiformis coinmunis. 



Fig. 3, line o, plate 2, which is the young of the PI. trochiformis, fig. 5, line r, has the 

 flattened apex and some of the characteristics of elegans Hilg., but also resembles the 

 discoideus involutus, and the discoideus rotundatus forms. From this, which often has 

 prominent carinations, we are able to pass into PI. trochiformis turhinatus Hilg., line s, 

 figs. 9-12. These forms have the flattened and sharply carinated young like those of the 

 less turbinate elegans Hilg., as shown on line a, fig. 15, plate 3, with a broken young speci- 

 men of PI. trochiformis (fig. 16) for companion. Compare then again with selected forms 

 of PI. discoideus in which the young are remarkably flattened and sharply carinated above 

 (as fig. 13, line c, plate 2, figs. 1-3), line k, figs. 12-14, line g, plate 1, and similar figures 

 of discoideiis on plate 3. The paucity of intermediate forms in this series is very marked, 

 and caused Dr. Hilgendorf, as in the case of rotundatus, to invert the natural order of the 

 series. He derives both of these forms from PI. trochiformis, whereas in tracing them out 

 as in the case of revertens it will, I think, be found that the young, which always indi- 

 cate the true line of descent, show closer afS.nity with discoidean than with trochiform 

 varieties. In this connection see figs. 6, 7, 8, line k, plate 1 (fig. 6 being very like PI. 

 elegans, though evidently intermediate between discoideus and trochiformis) ; also 5-10, 

 line h, plate 2, and then compare fig. 15, line a, plate 3, with flat discoideus above. The 

 resemblance of the young to these full grown forms will at once become apparent. So 

 far as my experience goes the only retrogressions from trochiformis,, which I have been 

 able to find, are like those figured on plate 8, which are diseased forms showing, as prev- 

 iously described, a tendency to depart from the spiral as in fig. 10, line r, and fig. 11,. line 

 s, plate 2. 



The conclusion from these observations seem to be that PI. trochiformis arises from the 

 almost simultaneous changes of a number of closely allied varieties. These are mostly 



1 The name of the species, Planorbis trochiformis, which ought to have been at the head of these remarks, has been 

 accidentally omitted on p. 78, above third paragraph. 



