x. ZONES OF AMMONITES. 133 



particular species have been selected to mark more than the three 

 or six zones here indicated, and with considerable success. In this 

 ammonitic chronology the first important step -was taken, half a 

 century since, by W. Smith, who composed a table of ammonites, 

 in natural groups, and referred them one by one to their appropriate 

 seats in the strata. But the table remained in MS. in the hands 

 of his amanuensis till 1860, when it was printed as part of the 

 address delivered by him as President of the Geological Society. 

 Von Buch in 1832 prepared a famous treatise on ammonitic cha- 

 racters and groups, and his results have had a great influence on 

 all subsequent efforts. In Germany, Professors Oppel and Quen- 

 stedt, in England, Dr. Wright, have been most successful in marking 

 out liassic ' zones ' by ammonitic species, and have in fact furnished 

 a * chronology' of great value, which, cautiously applied, helps and 

 will help in all investigations into even far-separated tracts of liassic 

 deposits e . 



The same method applied to belemnites yields very similar and 

 not less assured results f . 



It ought to be applied in all cases of large natural groups ; we 

 should have zones of Echinodermata, Brachiopoda, &c. treated 

 independently of the Cephalopoda, with which they would not 

 necessarily synchronize ; and finally all would be combined into a 

 true system of geological chronology. This is in progress. Mean- 

 time, in the following Table we have marked a series of ammonitic 

 species more or less limited to the several epochs, and in another 

 column appear other nearly contemporaneous forms of life. The 

 whole liassic period, comprising the Rtetic shales below and the 

 Cephalopoda! bed above, is included to make the view complete. 



e See in particular Oppel's Jura, Quenstedt's Jura, Wright's memoirs in Pal. Soc. 

 volumes. 



f See my memoir on Belemnites in Pal. Soc. volumes. 



