156 The American Geologist. septfiniwr, 1392 
Ammonites peculiar to each stratum and classified and defined 
successive zones therebj'. 
The name applied to each zone is the specific name of the Am- 
monite peculiar to the bed, as 
Zone of Amnionitt's {Aegoceras) platiorlns^ the planorl)is bed. 
" " '• '^ anguJdfns, angulatus bed. 
And so on, bncklandi, tubercuJufus, ohtnsus, etc., beds. 
Although this is not the purely comparative method, but is 
rather an extension of William Smiths principle of recognition of 
the beds by their fossils, the comparative element is seen in the 
succession of distinct species in succeeding comparatively thin 
beds. The studies of Branco and Hyatt expanded the investiga- 
tion to a comparative study of the series of Ammonites of a single 
genus, and brought out thereby the exact laws of succession of 
the several known representatives of the family and their relation 
to each other, showing unmistakable succession in series of forms, 
whose order can be accounted for only as genetic. 
Hilgendorf. in his famous study of the FldUDrhis of Steinheim 
and Waagen with the Ammonifrs, Neumayr with the PdJudinas, 
H{)rnes in the case of the Cdiicellaria, have traced elaborately the 
paleontological series of forms of a single genus, illustrating this 
important principle. 
In the case of the Pliocene Paliidinas examined by Neumayr, a 
series from below upward was traced, which at the base exhibited 
a normal Palndina (P. neinnayvi) and the latest of the series, {P. 
hoernesi) was not only regarded as specifically distinct, but as the 
type of a distinct genus, Tuhifomd. (See Neumayr, p. 57.) 
Prof. Hyatt has elaborately traced out all the known species of 
the Arictida in the same way, and arranged the different forms in 
series exhibiting their chronological mutations. Waagen applied 
this term '^Mufatiou'' to the modification of form observed on com- 
paring the successive representatives of such series, to distinguish 
it from the modifications which are exhibited contemporaneously, 
and are defined under the terms "variation" and "variety." The 
series of fossil horses described by Marsh and Huxley is another 
case in which the "mutations" reached a generic value. 
All through the field of paleontology may be found similar series 
of genera in whicli the succession is of such a nature as to suggest 
genetic relationship and to lead to the theoretical construction of 
phylogenetic lines of descent. 
