OF PLANORBIS AT STEINHEIM. 81 



look to the true discoideiis line for tlie 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 PL 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 dlscoideus. 



Others, however, equal to the variety pyrguUfonnls Sand., PL trochlf. 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, distinguishable from the true stilcatus 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 discoideiis. 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 k, plate 1. These appear to be identical 

 with Klein's V. midtiformis, var. trochiformis, and in part with Sandberger's mid(ifor7ms, 

 var. trochiformis communis. 



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

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

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

 prominent carinations, we are able to pass into PL 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 oi PL trochiformis (fig. 16) for companion. Compare then again with selected forms 

 of PL 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 discoideus 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 PL 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 affinity with discoidean than with trochiform 

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

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

 line b, 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 trochiforinis, 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 PL trochiformis arises from the 

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



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

 accidental!)' omitted on p. 78, above third paragraph. 



