THE GEOLOGIST. 



2. Passing over a non-fossiliferous bed of stiff clay, we 

 again find a bed of lias flagstone, in layers of about five 

 inches in thickness^ and of a brownish colour, the upper- 

 most layer of which abounds in small species of Mocliola and 

 Ostrea^ to which beds they seem almost entirely confined, 

 only a few scattered specimens being found in the lower 

 layers of the same bed, and none in the other forma- 

 tions. 



3. Another stratum of clay separates this from a third bed 

 of flagstones, in which Plagiostoma gigantea occurs. This 

 shell does not occur in any other bed of the lias formation 

 in this neighbourhood. 



4. Next in the ascending series, we find some thick beds 

 of lias clay, of variable colours, by which their existence 

 may be readily traced^ and the beds identified in all their 

 situations; whilst their fossil contents are also a ready 

 index to their identity. We observe new fossils occurring 

 in every bed, and occupying the places of those occurring in 

 the subjacent beds, so that the upper bed of lias clay con- 

 tains scarcely a fossil of the same species as the lower one. 



A. In the lowest bed we first find Gei^villia, and amongst 

 other genera, Feet en (2) Terebratiila (8) Nucula (2) and 

 Avicula (2). 



B. The next is a bed of stiff" blue clay about twenty feet 

 in thickness, abounding in Hippopodhim ponderosum, and 

 Lutraria, which are never found in a lower position in this 

 district. This bed contains occasional nodules of indurated 

 clay, w^hich often contain Ammonites, and a crustacean, 

 Astacus glabra. With regard to ammonites, they may be said 

 to commence in this bed, as we have never found one in- 

 dividual lower in the whole district to which this memoir 

 refers, and in this bed alone they are so numerous, that we 

 have been able to identify twenty species, among which the 

 following are those most abundant : 



