C 0 RBESPONDEN CE . 



5L 



species of mollusca, 6 only occur at Warminster, the remaining 30 Warmin- 

 ster species not being found at Blackdown. A further comparison of the 

 Blackdown fossil mollusca with those of the Upper Greensand generally, 

 and also with the fossil mollusca of the Gault, gave nearly the same result. 



These observations, combined with certain facts which came under my 

 notice while studying theLower Greensand of Surrey and the Isle of Wight, 

 led me, in the first place, to question the correctness of classing the Black- 

 down deposits with those of the Upper Greensand or even of the Gault; 

 and ultimately induced me to believe them, so far at least as the whet- 

 stone strata are concerned, to be exclusively of Lower Greensand origin — • 

 their position in the series being, probably, near the base of the upper or 

 ferruginous division of Fitton. 



It is, therefore, in the hope of stimulating further inquiries, which, for 

 myself, I have neither the time nor the opportunity to make, that I now 

 venture to address you on the subject, as I feel convinced that a closer 

 investigation of the lowest beds of the Greensaud at Blackdown must now 

 decide the question ; for, should the absence of Gault be confirmed, I think 

 the evidence of the fossils alone may be deemed conclusive. 



The following table shows the relation of the Blackdown fossils to those 

 of the Upper Greensaud, Gault, and Lower Greensand respectively : — 



Total number of Blackdown species (Mollusca) taken from ' Morris's 

 Catalogue, Fitton's List in Trans. Geol. Society, and from specimens in. 

 the British Museum and Jermyn Street Museum, 156. Of these, 81 are 

 peculiar to Blackdown ; 20 occur in Upper Greensand, 7 of which are li- 

 mited to Upper Greensand and Blackdown ; 16 occur in Gault, 4 of which 

 are limited to Gault and Blackdown ; 59 occur in Lower Greensand, 39 of 

 which are limited to Lower Greensand and Blackdown. 



The numbers for comparison, therefore, stand thus : — 



Species limited to Upper Greensand and Blackdown, 7 ; viz. — 



Ammonites denarius . . Not common (Sow.). 



A. falcatus A rare species (Sow.) 



Becten asper Abundant in G. S. near Frames 



Terebratula li/ra . . . Rare. 



Twrritella granulata . . Abundant at Blackdown ; U. G. S., lo- 

 cality doubtful. 



GrypTiaa vesiculosa . . Abundant in U. Gr. S., near Warminster ; 



Blackdown, doubtful. 

 Cucullcea carinata . , . Common at Blackdown; occurs in U.G.S., 



near Devizes. 



Species limited to Gault and Blackdown, 4 ; viz. — 



Ammonites auritus . . . Not common (Sow.). 

 A. tuberculatum .... 



Murex calcar .... Common at Blackdown ; other localities 

 doubtful. 



Hoslellaria calcarata . . Common at Blackdown and Folkestone. 



Species limited to Lower Greensand and Blackdown, 39; viz. — 

 Lower Greensaud locality. 



Astarte formosa . . . Shanklin Bare. 



A. obovata Sandown. 



Aricula Rauliniana . . Godalming. 



Cardium Hillanum . . Shanklin Frequent. 



Corbula truncata . . . Peasemarsh Common. 



C. elegans Peasemarsh Common. 



