162 



On the Ammonites of the Lias Formation. By Thomas Wright, M.D., 

 F.R.S.E., F.G.S. 



Before entering upon a description of the Ammonites of the Lias, I 

 shall give a short account of the different zones of life into which this 

 formation is now divided, with a view of defining more accurately the 

 distribution of the different species of this group in time and space ; and 

 likewise as an example of the value of Ammonites to the Palaeontologist 

 as indicators of time in the study of the secondary rocks. 



English geologists divide this foi-mation into Upper Lias, Marlstone, 

 and Lower Lias, but these sub-divisions require additions and modifications 

 in order to place the liassic beds of the British Isles in correct correlation 

 with those of France, Switzerland, and Germany. For on the TJj)per 

 Lias clays, in certain localities, are superimposed extensive arenaceous 

 deposits, which, previous to the publication of my Memoir on the Upper 

 Lias Sands,* were grouped with the Inferior Oolite, and in the Lower 

 Lias are included several beds of clays and marls which, with the Marl- 

 stone of English authors, form the Middle Lias of continental geologists. 



Taking the Lias beds so well exposed in their natural order of super- 

 position in the North and South of England, in the magnificent sections 

 on the Yorksliii'e and Dorsetshire coasts, and naming each group of beds 

 by the most characteristic Ammonite contained therein, we find the 

 following zones of life, taken in descending order : — 



* Palseontographical and Stratigraphical Relations of the so-called Sands of the 

 Inferior Oolite. — Quarterly Journal Geological Society, vol. xii., p. 292, 1856. 



