74 HYATT ON THE TERTIARY SPECIES 



The variety with the mouth turned downwards, in the normal direction of the spiral, 

 fig. 11, line m, pi. 1, has the third and fourth carinae well developed. Fig. 13, line h, pi. 1, 

 exhibits throughout its adult stage a form inseparable froni that described above, on 

 line n, fig. 1, but in old age the whorl is deflected in the normal direction. At the same 

 time the sulcation apjaears, and with it the first carina is indicated, and the second 

 carina becomes quite prominent and much better defined than I have seen it in any other 

 specimen. The aspect of the whorl, the decrease in its involution, and the enormous 

 comparative size of the shell show that these exaggerated features are the products of sen- 

 ility. A tendency to produce the second, third and fourth carinae is also observable in quite 

 a number of specimens with a decided and symmetrical flattening of the upper, outer, and 

 lower sides of the whorls, but no sulcations ; this is the case with fig. 16, line h, pi. 1, which 

 can only be separated from PI. tenuis, line c, fig. 2, by the absence of any linear sulcation 

 or depression on the upper side. 



Besides the above, there are still other modifications which it is necessary to mention. 

 These consist of exceptionally flattened forms. All of those previously described are more 

 or less stout, but there are some which do not partake of this characteristic, such as are 

 figured on line m, pi. 1, figs. 8-9. All, including the above just described, have rather wide 

 and open umbilici on the lower side, and cylindrical or flattened whorls, but there are 

 some forms, with stout whorls, in which this is exchanged for a tendency to narrow the 

 umbilicus, and these, though difficult to distinguish at first, are soon readily picked out 

 after a little practice. They are then seen to be distinct, and by comparison with PI. levis, 

 are recognizable as the intermediate forms of PI. ""^S™*? figs- 14, 16, line a, pi. .1. 



Flanorbis tenuis. 



Planorhis tenuis Hilg., Op. cit., fig. 3. 

 Carinifex tenuis Sand., Op. cit. 



V<»nV+-\r tenuis 

 ' dJ-icDj steinheimensis- 



Certain forms which 1 have included under this name are not distinguishable from certain 

 forms of Steinheimensis. They nearly all have faint but unquestionable marks of a 

 sulcation on the upper side, but so have some unquestionable specimens of PI. 

 Steinheime7isis, and that this is an artificial line can be readily shown by the comparison 

 of such specimens as fig. 16, line b, and fig. 2, line c, pi. 1. The latter I have often 

 referred to PI. Steinheimensis, when studying that species, because of its very faint 

 sulcations, and also on account of the extreme smoothness of the shell and its close 

 resemblance to true Steinheimensis forms, especially figs. 3, 4, line a, pi. 2. There is the 

 same story to be recounted with any characteristic which may be selected. 



First sub- variety. 

 This is represented by a flat shell with the first, third and fourth carinae indicated, but 

 the second almost absent. They are not distinguishable from such forms as fig. 14, line c, 

 pi. 1. They are rounded and smooth on the upper side, but flattened as in tenuis on the 

 lower side, with the third and fourth carinae well developed. Some of these have also 

 almost imperceptible sulcations on the upper side on the last whorl. These differences 

 disappear in fig. 13, same line, which is really a flattened Steinheimensis form with inter- 



