80 HYATT ON THE TERTIARY SPECIES 



to produce flattened forms with prominently developed carinations, but that this is 

 counteracted by a still stronger tendency to produce turretted forms, with the carinations 

 and sulcation of the lower side more or less depressed or atrophied, and that side 

 correspondingly more or less rounded. It will be seen at once that these tendencies 

 produce in one direction the squarest whorls and mouths, and in the other the 

 triangular or trochiform shape of the whorl and mouth. 



The specimen figured on line o, fig. 15, pi. 1, is the young of dlscoideus, with 

 a very close resemblance to the var. rotundatus, Hilg., differing only in the greater 

 angularity of the lower side. It is the young of a PL ''S^Ss' variety, and not of the true 

 PL trochiformis. There is one remark of importance to be made in this connection, that 

 this young form is intermediate between the flattest of the rotundatus-Yike young, 

 next to be described, and the young of the immature forms of dlscoideus, figured 

 below on line I, figs. 7-9. 



Var. rotundatus, as described and figured by Hilgendorf, is equivalent to the young 

 of several forms of PL trochiformis. 



Sandberger alludes to it as a full-grown variety of his intermedius, distinguished 

 by its smoothness, but in this sense it is not identical with Hilgendorf 's figure, which 

 relates to the young alone, as may be seen in the following remarks. 



If we compare the young of PL discoideus displayed on pi. Li, line i, figs. 1-9, and line 

 h, figs. 1-17, with the tenuis forms on line/, fig. 1-12, we shall first ascertain one very 

 important fact, which has been stated before : that the young of PL dlscoideus differ 

 in the smooth forms from the young of PL tenuis, in being stouter, even from a 

 very early period, though in all other respects they are identical. The discoideus series 

 is finished by the older specimens, figs. 13-17, line/. 



The continuity is unbroken on line h, and fig. 18 makes an easy transition with 

 its more or less rounded whorls, from dlscoideus to the forms of rotundatus. 



These do not entirely abort the third carina which is still to be seen in fig. 23, line h. 

 They do, however, inherit a tendency to become trochiform, at a very early age, 

 and with this a tendency to round the lower side of the whorl, and suppress the 

 third and fourth carinoe. If now we attempt to follow up the characteristics, we are 

 led into such forms as figs. 4, 5, 6, line p, pi. 2, with a linear third cariuation, which are 

 undoubtedly young of PL trochiformis, as figured on line q, fig. 18. 



The PL trochiformis, fig. 8, line n, is tracable in all its stages down to the vindoubted 

 rotundatus forms, fig. 12, same line, and figs. 15-18, line o. Figs. 9-10, line o, are 

 identical with PL discoideus, and at first seem really to belong to that species. They 

 bear nearly the same relation to that species as fig. 6, line c, differing only in having the 

 narrow umbilicus and peculiar sharpness of the lower whorls, which show that they are 

 the young of the same varieties of trochiformis as rotundatus. 



As a separate designation for a variety, the name rotundatus therefore has no 

 existence, unless accepted as noted by Sandberger, but even then it is open to this 

 difficulty, that very dissimilar varieties of PL trochiformis may be devoid of carinae. 

 They may be as distinct as the transition form, fig. 4, line r, pi. 2, and the PL 

 trochiformis, fig. 9, line k, pi. 1. 



This study of the young forms, however, leads to the conclusion that we must 



