12 HYATT ON THE TERTIARY SPECIES 



asymmetry of the spiral, and to introduce decisive sulcations and carinations, finally asserts 

 itself, and eventually produces representation. Thus, PI. supremus becomes very similar 

 to some of the varieties of PI. discoideus, and PI. *diScoiZus% particularly to those forms 

 which either retain or revert to the smooth character of the under side first observed 

 in PI. levis. This Third Series also fails in producing any modification comparable with 

 PI. trochiformis, fig. 7, for the same reason, because of the preponderance of its in- 

 herited peculiarities. Thus, although the last whorl is very asymmetrical, and quite 

 similar in shape to trochiformis, the young internal whorls are peculiar and always have an 

 upper umbilicus, and are therefore enveloped by, rather than elevated above, the outer 

 whorls. This condition is frequently excessive in PI. levis, and is the natural result of 

 the discoidal mode of growth during the early stages of the shell.-' 



These facts, when viewed as a whole, show that the progressive series are all more or 

 less parallel, and may be said in general terms to have the following succession of form, 

 First, a universal tendency to increase the spiral ; equivalent to the deepening of the lower 

 at the expense of the upper umbilicus, thus eventually producing more or less trochiform 

 shells. Second, the forms are modified in the following succession in each of these series, 

 starting from (1) the smooth forms, there occurs (2) a gradual increase in the flattening of 

 the sides of the whorl, both above and below, accompanied by (3) the introduction of 

 longitudinal sulcations and (4) carinations. 



There is, then, in spite of the diversity maintained by the differences, which have 

 arisen in the original progenitors, and which continue to be inherited throughout all 

 the members of each genetic series, certain uniform tendencies which have led to the 

 genesis of certain similarities in the form and even in the minor characteristics of the 

 species in different series. Farther than this, it may also be said that these uniform ten- 

 dencies, as expressed in the spirality of the growth and the introduction of new character- 

 sties, have a certain uniform succession. 



The phenomena, therefore, indicate the action of some general cause which controls the 

 tendency to variation first observed in the varieties of PL levis, and brings about a certain 

 uniformity in the production of forms and representative characteristics in each progressive 

 series. 



The First Series and its sub-series are also subject to the control of the same law 

 so far as the tendency to increased spirality is concerned, but in other respects they 

 deserve special consideration. The similarities of the lower representative forms, figs. 

 25, 21, 18, are evidently accounted for in somewhat the same way as those of the 

 progressive, viz. : they are close to the point of origin. Unlike these, however, they 

 are so nearly related, that the hybrids or intermediate forms are numerous. It will 

 be observed that they possess the flat form of whorl also found in the Second 

 Series. We are on this account able to compare figs. 25, 21, with 13, and PI. cos- 

 tatus, fig. 26, with PL crescens, fig. 14. The representation of PL major, fig. 27, 

 and PL ^Zinutmi fig- 22, is also evidently due to the close affinity of the two, and the 

 tendency of the whorls to become uncoiled and to degenerate in outline gener- 

 ally, as may be seen by observing the apertures of the shells. Fig. 28, PL costatus 



'There are, however, forms much more Uke trochiformis line 1, fig. 4-11, but of these no full grown ones seemed 

 than fig. 11, see PL oxystomus, var. rotundatifomiis, pi. 3, any closer than the one figured. 



