OF PLANORBIS AT STEINHEIM. 13 



var. distortus, and PI. *""«?«"* ■> fig- 23, can be explained in the same way. Var. denudatus, 

 fig. 24, crowns one sub-series standing alone, as did also the remarkable PI. troehiformis, 

 the latter as the extreme of the progressive series, and the former as the extreme of 

 the retrogressive series. 



These sub-series are not all purely retrogressive. The second sub-series is almost wholly 

 so, because it does not add a single new character to those observed in PI. "","m'r ? except 

 the tendency to form a spiral. It goes steadily without a break, from the closest coiled, 

 smooth, dLscoidal form of the latter, to the extreme spiral, trochiform, and partly 

 uncoiled denudatus, and remains throughout diminutive in size, smooth and with a 

 cylindrical and extremely embryonic form of the whorl. The first sub-series, however, 

 while it agrees entirely with the second in the size and form of the whorl and shell, 

 and the tendency to increased spirality, nevertheless adds a new characteristic, the 

 enlarged transverse striations or ribs, and increases in size in some species, as in figs. 

 26 and 27. 



Thus only the second sub-series is almost entirely retrogressive, and yet both the first 

 and second have diminutive shells, and the first has also disea.sed forms, which present a 

 tendency to uncoil the shell. This last characteristic is only observable in isolated instances 

 in the species of the purely progressive series. Thus it occurs as figured on pi. 8, line a, 

 fig. 1, to an incomplete degree in the extreme old age of PI. Steinheimensis ; and for a 

 similar reason in a very large PI. tenuis, pi. 1, line k, fig. 11 ; in PI. discoideus, in different 

 degrees, pi. 1, line g, fig. 10 ; line i, fig. 6 ; as a pathological condition of the individual 

 either due to wounds, disease or premature old age ; in PI. oxysiomus to a most extraordi- 

 nary degree as figured by Sandberger, and to a less degree in figs. 7, 8, 9, line p, 

 pi. 3. These instances, however, are very instructive, since in fig. 8, the scars of severe 

 wounds are apparent on the shells, whereas figs. 7 and 10 exhibit no cicatrices, and are 

 evidently the result of some weakness caused by disease in the animal ; in PL troehiformis, 

 pi. 2, line r, fig. 10, and other specimens as previously described, it occurs as the result 

 probably of some disease. PI. 8 is especially devoted to these senile and diseased speci- 

 mens, which will be described more fidly further on. 



The uncoiling of the whorls must therefore be looked upon as a sign of weakness in the 

 animal, and as the result of pathological conditions, whether these be normal as in the final 

 retrograde transformations of advanced senility and disease of any kind affecting the 

 adults and young ; or traumatic and abnormal as in the cases cited where wounds and other 

 accidents may have caused disease in the animal, followed by a weak condition in which the 

 usual increase of the shell by growth could not be maintained. Therefore there is the 

 strongest reason for calling the second sub-series a purely retrogressive series, and the first 

 partly retrogressive, since not only do they show retrogression by the size of the species, and 

 their failure to produce comparable series of new and varied forms with newly introduced 

 characteristics, but they show common variations, which can be compared with the patho- 

 logical variations and metamorphoses of the shells of individuals of the progressive series. 

 It will be observed by all who read this memoir attentively, and study the plates especially, 

 that these uncoiled forms do not occur, except in the cylindi'ical whorled forms which are 

 transitional from PL minutus to denudatus and distortus, and this cylindrical characteristic 

 is decisively retrogressive. It can occur only in those individuals which do not inherit 



