54 GENESIS OF THE ARIETIDA. 
I. 
GENEALOGY. 
GENERAL REMARKS. 
HE Arietidx are divisible into three parts or stocks, the Psiloceran, the Pli- 
catus, and the Levis Stocks. Each of the last two were probably derived 
from different varieties of the single species Psi/. planorbe, or its geographical 
affine in the province of the Mediterranean, Psi. caliphyllum' Psiloceras can 
consequently be appropriately designated as the Radical Stock of the Arietide, 
and, as stated above, this genus is also a surviving member of the radical stock 
of the whole order of the Ammonoidea. It may also be considered as the first 
branch of the Arietide. 
The Plicatus Stock has four genera or series, Weehneroceras, Schlotheimia, 
Caloceras, and Vermiceras. The first and second depart widely from the normal 
Arietide, and they can be considered together, as if forming a second dis- 
tinct branch. Caloceras and Vermiceras, especially the latter, are distinctly 
arietian in aspect, and may be classed together in another or third branch 
of the family tree. 
The Levis Stock has five genera or series, Arnioceras, Coroniceras, Agassi- 
ceras, Asteroceras, and Oxynoticeras. The first two are derivatives of the same 
radical species, and can be closely associated as a fourth branch of the family. 
Agassiceras and Asteroceras cannot be so closely associated with Oxynoticeras, on 
account of the wider divergence of the adult characters of the last genus, but 
they are apparently derivatives of the same radical species, and can therefore be 
joined, forming a fifth branch. Oxynoticeras thus becomes separated from the 
rest of the Arietide: as a sixth branch composed of one genus. 
The Plicatus Stock can be followed from Psiloceras into a series of forms hav- 
ing transitional characters, Weehneroceras, and ending in the production of the 
peculiar series Schlotheimia, having characteristics widely divergent from those 
of the normal forms of the Arietidse in their pilse, and in their single-channelled 
keeless abdomens. The sutures retained the peculiar phylliform or psiloceran 
character. In another direction, the same stock built up in part the normal Arie- 
tide, producing by gradual modification the vermiceran series. Caloceras, though 
truly arietian in aspect, was nevertheless much like Psiloceras, especially in its 
sutures. The latter lost their complicated psiloceran characters in Vermiceras, 
and became simpler in outline, or typically arietian. 
The Levis Stock had no such complete transitions to Psiloceras, and began its 
modifications at a later time in the Lower Lias, springing at once into the true 
1 See Summary Plates xi. to xiv., which should be studied in connection with these ‘‘ General Remarks.’’ 
