COEBESPONDENCE. 
51 
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 Fittou. 
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 t)ie Greensand at Blackdown must now 
decide the question ; for, should the absence of Gault be confirmed, I think 
the evidence t>f the fossils alone may be deemed conclusive. 
The following table shows the relation of the Blackdown fossils to those 
of the Upper Greensand, 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 ZTpper Greensand and Blackdotcn, 7 ; viz. — 
Ammonites denarius . . Not common (Sow.). 
A.falcatus A rare species (Sow.) 
Fecien asper Abundant in G. S. near Fromo. 
TerehratiUa lyra . . . Kare. 
Tarritella granulata . . Abundant at Blackdown ; U. G. S., lo- 
cahty doubtful. 
OryplicBa vesiculosa . . Abundant in U. G-. S., near Warminster; 
• Blackdown, doubtfid. 
Cucullcea carinata . . . Common at Blackdown joccui's in U.G.S., 
near Devizes. 
Species limited to Gault and £lackdoicn, 4 ; viz. — 
Ammonites am-itus . . . Not common (Sow.). 
A. tuberculatus .... 
Murex calcar .... Common at Blackdo^ii ; other localities 
doubtful. 
Rostellaria calearata . . Common at Blackdown and Folkestone. 
Species limited to Loioer Greensand and Blackdown, 39 ; viz. — 
Lower Greensand locality. 
Astarte formosa . . . Shanklin Eare. 
A. obovata Sandowu. 
Avicula Rauliniana . . Grodalmiug. 
Cardium Hillanum . . Shanklin Frequent. 
Corbula truncata . . . Peasemarsh Common. 
C. elegans Peasemarsh Common. 
