60 HYATT ON THE TERTIARY SPECIES 



exceptions to this law of correllation, since in sub-series second, the Avhorls become less in 

 respect to their breadth, or more cylindrical as has been previously stated. It will also be 

 seen by looking at pi. 4, that there is a decided increase in size iii the costate sub-series, 

 as was to be anticipated in correllation with the partially progressive characteristic of the 

 well-marked costae which appear in this sub-series. The contrast between the large and 

 comparatively healthy specimens of this sub-series and the smaller distorted ones, is very 

 well shown also on lines h and k, as contrasted with g and i. This tendency to such 

 correllations as here described, show that the varieties of each species are quite closely 

 parallel with the general progress or morphology of the most advanced group, the Fourth 

 Series. 



Thus not only does every species vary from a more or less aequiumbilicated to a more 

 or less asymmetrical rotundatus or /rocAJ/brm-like shell, but the whole series of 

 changes in form of the Fourth Series is similar. See pi. 9, figs. 1-7. 



To this I might add if space permitted, many otlier e.xamples among the fossil 

 Ammonites and living animals. In fact, in my experience, the general limits of 

 variation are indicated in the range of form of almost any numerously represented 

 species of a group. The difference between the morphological range in a species of this 

 kind and the group to which it belongs being one of degree, one of quantity rather than 

 quality,' or otherwise there could be no parallelism between the morphological variations 

 of form in the species and the series of forms, which are comprised in the group to which 

 the species belongs. 



Planorbis triquetrus. 



Planorhis imiUiformis triquetrus Hilg., Op. cit., fig. 17. 



Var. typica. 



Fig. 18, 2^1. 9, leads into shells, fig. 9-23, line g, pi. 2, in which the upper side becomes 

 slightly channelled, and sometimes the lower side also, as in Hilgendorf's figure. 

 We are here presented with a remarkably close parallelism with the thinner forms 

 of PL tenuis, but it is not very difficult to separate even large perfect forms of 

 PI. triquetrus from the young of forms which are figured in the two lines below on the 

 same plate. 



Var. turhinatus. This includes three specimens, which, as in figs. 20, pi. 9, or figs. 5, 6, 

 9, pi. 4, line h, become more trochiform than var. typ)ica. 



Planorbis denudatus. 

 PI. multiformis denudatus Hilg., Op. cit., fig. 19. 



Hilgendorf's arrangement of the derivative forms which may be designated by this 

 name, appears to be defective, in so far as he traces the uncoiled, smooth or '*' denudatus'' 

 variety, to the coiled and round whorled " costatus." My collection gives a perfect 

 series, without break of any kind, from the perfectly smooth PL 7nimifus to a 

 completely asymmetrical shell, which differs but slightly from Hilgendorf's figure 

 of PL denudatus. 



' It must always, however, be understood that parasites supreme modifier, and the reaction of the organization 



do not come under this law, nor any range of forms however against the environment, and the maintenance of the type by 



closely connected, which have been placed in e.xceptional this reaction, cannot survive in the presence of continuously 



suiToundings. As previously stated, the environment is the exceptional surroundings. 



