54 GENESIS OF THE AKIETID.E. 



II. 



GENEALOGY. 



General Remakes. 



THE Arietidae 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 Psil. planorbe, or its geographical 

 affine in the province of the Meditei'ranean, Psil. caliphjllum} Psiloceras can 

 consequently be appropriately designated as the Radical Stock of the Arietidae, 

 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 Arietidae. 



The Plicatus Stock has four genera or series, Waehneroceras, Schlotheimia, 

 Caloceras, and Vermiceras. The first and second depart widely from the normal 

 Arietidae, 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 Arietidae 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, Waehneroceras, and ending in the production of the 

 peculiar series Schlotheimia, having characteristics widely divergent from those 

 of the normal forms of the Arietidae in their pilae, 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- 

 tidaa, 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 si. toxiv., which should be studied in connection with these " General Remarks." 



