1 
f 
4 
22 GENESIS OF THE ARIETIDA. 
convergent outwardly, and the abdomen narrower, though the shell may still re- 
main discoidal; ex. Caloceras and Coroniceras, Plate XIV. Fig. 11-16, 28-32. 
The Ammonoids of the Lias also have a tendency to produce keels, ribs, etc. in 
addition to the parallel procession of the forms just described. Thus, when we study 
the parallelisms occurring in different series or genera of the Ammonitine in the 
same family or group, we find that equivalent species in different series are due 
not only to the increasing involution of the whorls, but also to the development 
of similar structural characteristics. Most paleontologists are not aware of these 
facts, and therefore are apt to consider species of distinct series as closely allied. 
It is usual, for example, to classify all the species of the Arietidee having quadrago- 
nal whorls, deep channels, prominent keels, and well developed pile, as species 
of the same genus, Arietites,’ whereas they are more closely allied to Psil. 
planorbe, their radical ancestor, than they are to each other. Errors of this kind 
are common, and have been still more general. Thus most modern improve- 
ments in taxonomy in all the branches of the animal kingdom have consisted 
in doing away with classifications made by the association of representative 
forms, or, as they are here called, morphological equivalents. 
The Arietidx sprang from discoidal species of Psiloceras, having smooth shells 
and phylliform sutures. Other groups occurring later in time are traceable to 
forms of more advanced structure, so far as the shape and ornaments of the 
whorl and the sutures are concerned. In every case, however, progressive groups 
have been traced directly to forms having discoidal shells. The discoidal radicals 
of different series have been invariably found to be nearly related to each other, 
and to preceding discoidal radical types, while their descendent species are diver- 
gent, and essentially distinct. However closely they might have resembled each 
other as morphological equivalents, they possessed the homogenous differential 
characteristics of their own genetic series. 
I have elsewhere noted the facts tending to establish the probable existence 
of a continuous line or radical stock of types or species.? The paleozoic primary 
radicals are similar to Anarcestes; the mesozoic or secondary radicals are like 
Dinarites Mahomedanus, Ceratites Sturt, Gymnites, and Psiloceras; they occur largely 
in the Trias, and are species with discoidal but rather compressed smooth shells. 
The tertiary radicals, though discoidal, may be highly ornamented with pile and 
spines, and have sometimes very broad or coronate whorls; they occur largely 
in the Jura.? The primary and secondary radicals, if we follow Haeckel’s nomen- 
1 Zittel’s Handbuch d. Pal., I. p. 455. 
2 Gen. of Foss. Ceph., Proc. Bost. Soc. Nat. Hist., XXIII., 1883, pp. 823-825. 
8 Tirolites and Tropites are acmic or tertiary radicals occurring in the Trias. They are certainly coro- 
nate forms, with pil, tubercles, and open umbilici. If any one will compare the young of Balatonites or 
Tropites with the adults of the smooth species of Dinarites and Ceratites as figured by Mojsisovics, he will 
be able to see that the radical stock is a definable series of forms, with characteristics not only shown in the 
adults of simpler smooth genera and species, but necessarily repeated in the young of more modified species, 
like Balatonites, Tropites, etc. It must be remembered, however, that all forms will not have the smooth, 
compressed secondary radical reproduced in their young; many of them lost this, or had it only very 
slightly, since it was replaced by the broader-abdomened tuberculated tertiary radical, as in the young of 
Trachyceras aon. The young of Tropites has a form and sutures similar to those of Glyphioceras diadema 
of the Carboniferous, and the stock of tertiary radicals may therefore be said to have begun even in the 
Paleozoic. 
