84 HYATT ON THE TERTIARY SPECIES 



The shells are more magnified than in the first three plates, and carefully selected 

 to show their affinities. 



Line a, fig. 1, PI. levis, from Undorf, compares with figs. 3, 5, PI. Steinheimensis . 

 The latter have peculiarly deep umbilici on the upper side, which is a characteristic of 

 PI. levis, and are otherwise just like fig. 1 ; the forms could not be separated if found 

 together. Fig. 2 shows the typical form of PI. levis, from Undorf Fig. 6, also, from 

 Undorf, exhibits the angular whorl of PI. levis, and compares equally closely with fig. 7, 

 PI. Steinheimensis and fig. 8, a young specimen of the latter. Compare also fig. 2, line 6, 

 from the Pits, with PI. levis, Undorf, fig. 3 ; fig. 1, line h, PI. Steinheimensis, with fig. 4, 

 PI. levis, Undorf; figs. 5, 7, 8, line h, from the Pits, with figures on the line below, and 

 fig. 6, from Undorf. It is observable, however, that while in PI. levis a decided angu- 

 larity is found in the outer edge of the mouth-rim, owing to the prominence of the third 

 carination, this is not the case with Steinheimensis, as a rule. It is a rare exception in 

 the last named, and none of the norma] specimens examined by me, even though they 

 had this third carination as strongly shown as in fig. 6, line a, plate 7, possessed an 

 angulated aperture. 



Fig. 3, line c, shows a remarkably fine specimen of PL Steinheimensis, which has the 

 cylindrical form of the whorls common in var. aequiumbilicatus HUg., but is not aequium- 

 bilicated. It is decidedly asymmetrical, and shows the tendency to form a turbinate 

 whorl, which is so common in all the forms. Line c exhibits forms of Steinheimensis 

 intermediate with regard to the stoutness of the whorl, etc., between those in the three 

 lower lines, and line d. These specimens also show, as dd those below, that in some 

 specimens the same variations occur also in the young. Line e, figs. 1-4, are more or 

 less symmetrical forms, all normal, healthy, full-grown, but not outgrown, or old speci- 

 mens, as in fig. 13, line h, pi. 1. Fig, 7, line d, is PI. steinhei^Lsisj from the rocks of the 

 Upper Tier of the Cloister Ridge ; figs. 5-7, line e, are PI. stemTei^nsis from the Pits. 



Figs. 1-3, line/, complete the transition to PI. tenuis, figs. 4-5. 



The asymetrical forms of teimis are shown in fig. 6-7, same line, and on line g, 

 all of which are tenuis forms. Fig. 6, line g, is the stoutest true tenuis form which 

 I have been able to find in my collection, and it will be seen that a hair's breadth 

 more and a shade more of prominence to the third carina would make it impossible 

 to say whether it was tenuis or discoideus. 



The remaining specimens on this plate exhibit the ordinary forms of discoideus, those 

 having rotundatus young. Figs. 3-6, line i, are true rotundatus Hilg., and figs. 1, 2, 

 same line, the transitions from PI. discoideus to these. 



It remains now only to trace the law of acceleration in this series. If we select any 

 of the transition forms, PI. steiSmemis, and examine the younger stages, we should find, 

 as in fig. 1, line b, fig. 13, line c, pi. 1, that the only traceable resemblance to PL tenuis, 

 the sulcation, occurs on the last part of the last whorl, or during the full grown adult 

 condition only ; that such shells as figs. 14-15, line c, exhibit the same at an earlier age, 

 accompanied by carinae, and finally fig. 16, at a still earlier age ; so that it is difficult to 

 say whether the young is like PI. Steinheimensis, except at a very early stage. 



Finally in many specimens of PI. tenuis it becomes difficult to recognize the form of 

 PI. Steinheimensis at all on account of the early period at which the flattened whorl, 



