OP PLANORBIS AT STEINHEIM. "79 



line p, fig. 12, so excessively altered, that it is difficult to recognize the same variety. 

 But observe closely the shape of the inner whorl in this and in fig. 13, then it will 

 be seen that these are flat on the lower sid6, and in all respects similar in shape to fig. 3, 

 line i. 



Still another form of this variety is shown on pi. 2, line h, fig. 1-4, in which the 

 turretted forms are not so, and may be traced directly back to the thinner forms 

 mentioned above, through fig. 6, line i, pi. 1. 



Within the limits of these series, there are a great many varieties which appear to be 

 simply individual, or else to obtain only in a limited number of individuals, and to have 

 no special meaning. Such is the appearance of an anomalous carination, along the centre 

 of the whorl, as in fig. 5, line Tc, pi. 1. The existence of this appears to me unac- 

 countable, as a normal characteristic, and it is not perpetuated. The peculiar mode 

 of growth shown in figs. 17-19, line e, and fig. 18, line d, pi. 1, in which the reg- 

 ularity of the spiral is abandoned, seems to be another characteristic equally unparal- 

 leled. 



Besides these anomalies, there are numerous individual characteristics appearing with 

 exceedingly limited range, which can be better accounted for. These are largely mere 

 excessive developments of certain parts, as certain carinations, or suppression of others. 

 Very often these have a meaning, which is very important. They are often really char- 

 acteristic of a series, as the extraordinary development of the first carina in a large num- 

 ber of the sulcatus series. Again they may be of more limited application, but even more 

 significant and instructive. 



If we observe any series of square forms, such as 7-14, line g, pi. 1, there will be 

 some with the upper and lower sides of nearly equal breadth, and others in which the 

 lower side is narrower, never broader. In such extremely square forms as no. 19, line/, 

 and fig. 11-14, line i, pi. 2, this is quite as well marked as in other series, fig. 10-11, line c, 

 pi. 2, being the young of such forms, showing the the narrowness, as does also fig. 13, 

 line i. Sometimes this tendency is accompanied by a tendency to suppress the carinations 

 on the lower side. When carried to its extreme, as in the series figs. 1-6, line 

 c, this produces a whorl, which is rounded and smooth on the lower side, . as in 

 fig. 6. If we now compare these forms with line k, fig. 6,^ pi. 1, we shall see that 

 this is one of those transitions to PI. trochiformis, which are distinguished by having 

 smooth lower sides. 



In the same way, where the carinae are not atrophied, it wiU be observed that the 

 narrowing of the lower side always takes place in the transition forms to PL trochiformis 

 in proportion as they become more turretted. 



I do not mean to trace out a series on this character, because no one can examine 

 a series without seeing that the na/rowing of the lower side and the sometimes correlative 

 atrophy of the ribs is not characteristic of series, but is an individual variation, gaining in 

 importance and represented in a larger number of individuals, as we approach the 

 turretted forms. See also in this respect the transition forms, figs. 1-4, line r, pi. 2. We 

 can say also, with approximate accuracy, that in any series there is a constant tendency 



1 The PL elegans of Hilgendorf. 



