3M 



UPPER AND LOWER CRETACEOUS ROCKS. [Gh. XVIII. 



Avicula ; also the remarkable shell Diceras Lonsdalii, eminently 

 characteristic of the ferruginous beds of the Lower Greensand in 



Fig. 333. 



Geroillia aneeps, Desh. . 

 Lower Greensand. 



Fig. 334. 



Fig. 335. 



Trigonia eaudata, Agass. 

 Lower Greensand. 



Terebratula sella, 



Sow. Lower 



Greensand. 



Wilts. This genus is closely allied to Chama, and the cast of the 

 interior has been compared to the horns of a goat. The same shell 

 has been referred by some authors to Caprotina, and by others to 

 Requienia. 



Fig. 336. 



Diceras Lonsdalii. 

 a. The bivalve shell. 



Lower Greensand, Wilts. 

 5. Cast of the same. 



Palmontological relation of the Upper and Lower Cretaceous RocTcs. 

 — Professor Ramsay has deduced from an analysis of tables drawn up 

 by Mr. Etheridge of the fossilsof the Cretaceous series of Great Britain 

 the following conclusions : — First, that a great number of species are 

 common to the different subdivisions of the Upper Cretaceous group, 

 such as the Gault, Upper Greensand, White Chalk, &c. 



Secondly, that there is a great break between the Lower and Upper 

 Cretaceous series, for of 280 species of all kinds of animal remains 

 known in the Lower Cretaceous, 233 are peculiar, and 51, or only 

 about 18 per cent., pass from the Lower Greensand to the Gault and 

 overlying strata. 



The same geologist adds : " This break and disappearance of so 

 many species in succession is accompanied by a stratigraphical break 

 as well ; for round the Weald it is known that in some of the very 

 few exposures of junctions the Gault has been seen lying on eroded 

 surfaces of Lower Greensand, while in the western and middle parts 

 of England, on the west and north of the great chalk escarpment, 

 the frequent and sudden overlaps of the Lower Greensand by the 



