OF PLANORBIS AT STEINHEIM. 73 



This species or variety may be distinguished from the preceding, principally by the 

 more rapid increase in the size of the whorls by growth, and the consequently slightly 

 greater breadth and involution of the last whorls. The young are habitually asymmet- 

 rical, as in the section fig. 7, line a, pi. 1. No absolute line can, however, be drawn 

 between this and the preceding. The varieties are very numerous, and I cannot pretend 

 to enumerate all of them. The most significant, if it may be so expressed, are the 

 following : 



1. Those with the mouth turned upwards, or against the spiral ; line 6, fig. 3, pi. 1. 



2. Those with the mouth central ; line a, fig. 9, pi. 1. 



3. Those with the mouth turned downwards ; line m, fig. 13, pi. 1. 



All three of these varieties may occur with perfectly rotund, smooth whorls, without 

 the slightest indications of a carina-like angularity or ridge, in any light, or in any 

 position in which they may be held. There is, however, this observable peculiarity : the 

 larger number of carinated specimens have the mouth central or turned downwards, 

 rarely deflected against the spiral. 



All three of these principal varieties may also occur with the carinations described in 

 the preceding form, and in one case a distinct third carina appeared, fig. 1, line n, pi. 1, 

 which will presently be described in detail. 



Specimens with the mouth turned upwards, have a tendency to flatten the upper side 

 of the whorls. The second carina is very rarely indicated in these, but the third very 

 often. Sometimes it is alone, but usually it is accompanied by a well defined tendency to 

 flatten the lower side, and produce the fourth carina also, as in fig. 3, line b, pi. 1. 



The same peculiarities, word for word, may be attriljuted also to those having the 

 mouth central. Those figured on line b, figs. 6-10, pi. 1., have only one carina, 

 the third, indicated as in the mouth of fig. 6, inside of which it is well shown, though 

 rather too delicate and unpronounced to be shown in others. Figs. 4, 5, have the 

 third and fourth carinae indicated, especially the specimen shown in fig. 4. Fig. 4, 

 line n, plate 1, exhibits this peculiarity more markedly than it can usually be shown 

 by photography. 



In this variety also a sulcation makes its appearance as a fxint depression on the 

 upper side, as shown in fig. 2, 5, line n, pi. 1. The extraordinary form, figured on line n, 

 pi. 1, fig. 1, not only has the third and fourth carinae, but shows a peculiarly broad, dorsal 

 aspect, and has indications of the second carina, and a very slight depression or sulcation 

 on the upper side, quite equal to that in No. 5, on the same line, though hidden by the 

 flare of the mouth. These characteristics are well marked, according to the usual fashion 

 in other specimens, on the last whorl, for about three-fourths of its length. A con- 

 striction occurs in this specimen, caused evidently by some accident to the shell, which 

 was probably the immediate cause of the appearance of these unusual characteristics. 

 After the repair of this injury, the third carina appears as a regular ridge with a definite 

 linear depression on either side. 



The old age of Stehihehnensis is indicated in this and other large specimens by a 

 slight decrease in the amount of involution of the last quarter of the last whorl, and this 

 pecuharity is well marked in this shell, so that the distinct carination appears to arise, in 

 the old age of this form. 



