THE LIAS AMMONITES 



OF THE 



BRITISH ISLANDS. 



Before describing the Ammonitidse of the Lias I purpose giving a short account of 

 the several zones of life into which this great formation is divided, with the view of 

 defining their statigraphical distribution in time and space. More ample details on the 

 subject will be found in the various works cited in the synonyms prefixed to the 

 description of the different zones. 



English geologists divide this formation into Upper Lias, Marlstone, and Lower Lias, 

 but these subdivisions require additions and modifications in order to place the liassic 

 beds of the British Isles in strict correlation with those of France, Switzerland, and 

 Germany. For on the Upper Lias clays, in certain localities, are superimposed extensive 

 arenaceous deposits, which, previous to the publication of my memoir on the Upper Lias 

 Sands, 1 were grouped with the Inferior Oolite, and in the Lower Lias are included 

 several beds of clays and marls which, with the Marlstone of English authors, form the 

 Middle Lias of Continental geologists. 



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

 north and south of England in the magnificent sections on the Yorkshire and Dorsetshire 

 coasts, and naming each group of beds by the most characteristic Ammonite form 

 contained therein, we find the following zones of life, taken in descending order : 



The Upper Lias. — The sands 2 forming the upper portion of this zone, are charac- 

 terised for the most part by Ammonites belonging to the groups Harpoceras and Lyto- 

 ceras, as Harpoceras opalinum, Rein., H. radians, Schloth., H. Thouarsense, d'Orb., 



1 " Palseontographical and Stratigraphical Relations of the so-called ' Sands of the Inferior Oolite,' " 

 ' Quart. Journ. of the Geological Society,' vol. xii, p. 292, 1856. 



3 Midford Sands, so named by the late Professor Phillips in his ' Geology of Oxford and the Thames 

 Valley,' p. 118, 1871. 



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