78 HYATT ON THE TERTIARY SPECIES 



Variety discoideus. 



The sidcatus form is simply a compressed variety of PI. dlscoideiis with a remarkably 

 square mouth and whorl as in figs. 7-14, line g, line h, and 8-10, line p, pi. 1. The square- 

 ness and angularity of the under side is well depicted in figs. 2-3, and 7-9, line h. There 

 is a prevalence of these forms in formation a, but they are accompanied by true disco- 

 ideus, and every attempt to separate the two has been attended by great confusion in my 

 own mind. 



The typical forms of PL discoideus invohdus Hilg. are shown in figs. 10-17, line/, and 

 1-6, line g. I have applied this name to still more involute and more easily distinguish- 

 able forms, the extremes of this variety. 



Figs. 4-7, line g, show a still stouter form in which carinations and sulcations are more 

 marked, but the mouth at an early age has not the angularity of PL discoideus ; figs. 

 14-17, line /, the carinations are still more distinct, and the mouth is angular in the 

 young. Figs. 10-13, line f, show a trifle stouter and more trochiform shell with per- 

 fectly developed carinae, and sulcations both above and below. 



After this, the difficulty in following the series consists only in settling the affinities of 

 the adults of the numerous varieties ; the young remain quite similar in form, though 

 differing greatly in being more or less carinated or sulcated. They accord in this respect 

 usually with the adults, as for example, fig. 7-14, line/, pi. 1, and the various series of lines 

 d, e, / pi. 3. In this way it may be shown that the species really most closely related 

 to PL tenuis are the somewhat flattened but trochiform and unconnected forms like fig. 7, 

 line I, pi. 1, and not the deeply sulcated and carinated forms like sidcatus, however 

 flat they may be. 



The first variation is that cited above, in which the upper sides of the whorls have the 

 first carina well developed, the second rounded oS, with the third and fourth on the lower 

 side prominent. This leads into a number of allied forms, such as figs. 9, 10, line e, 

 pi. 1, which become very turretted, and on the other hand into exceedingly flat forms 

 like fig. 4, same line, in which all four carinae are well developed, but the sulcations not 

 well marked. Figs. 15-18, pi. 2, line /, shows a similar series of modifications, and these 

 are evidently the young of the trochiformis-like varieties, on line n, figs. 3, 4, and line m, 

 figs. 3, 4, and which lead in '"i^s'coukul^ same line, figs. 1, 2. 



A series might also be formed with the variety elatior Sand, as a type, fig. 16, line i, 

 pi. 2. This also, it seems to me, is not sufficiently described when included with the 

 discoideus series, and placed as Sandberger places it among the transition forms from 

 true discoideus, to his variety intermedius, sub-variety communis, figs. 1-2, line in. 

 Though quite closely allied to this series, there is a certain outline to the mouth, and 

 an aspect of the whorl, which is reproduced in trochiformis, figs. 6, 7, line r, pi. 1, and a 

 series could be doubtless formed connecting the two. 



Going back again we may take up almost any other line of characteristics, and follow 

 them out to a similar result. ^ 



Let us take, for instance, the true sidcatus form, already studied in part. In this we 

 find the flattened tenuis-like form, line A, pi. 1, figs. 10-12, and \h\Q jh fig'*- 1? 2, becoming 

 stouter, 3-10, same line. On line h, figs. 2, 3, we can see them more trochiform, and on 



