22 GENESIS OF THE AEIETIDiE. 



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 Ammonitinse in the 

 same family or group, w r e 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 palaeontologists 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 Arietidae having quadrago- 

 nal whorls, deep channels, prominent keels, and well developed pilce, as species 

 of the same genus, Arietites, 1 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 Arietidaa 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. 2 The paleozoic primary 

 radicals are similar to Anarcestes ; the mesozoic or secondary radicals are like 

 Dinariles Mahoniedanus, Ceratiles Sturi, 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 pilae and 

 spines, and have sometimes very broad or coronate whorls ; they occur largely 

 in the Jura. 3 The primary and secondary radicals, if we follow Haeckel's nomen- 



1 Zittel's Haudbuch d. Pal., I. p. 455. 



2 Gen. of Foss. Ceph.. Proc. Bost. Soc. Nat. Hist., XXIII., 1883, pp 323-325. 



3 Tirolites and Tropites are acmic or tertiary radicals occurring in the Trias. They are certainly coro- 

 nate forms, with pike, 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. 



