408 PROCEEDINGS OF THE GEOLOGICAL society.  [ Mar. 23, 
Dimensions. Length 0°35 inch; breadth 0:25 inch; thickness 
0-25 inch. 
Geological position. Zone of Ammonites Jamesoni: Cheltenham 
(%. 7.); zone of Am. margaritatus, Hinterweiler, Swabia (Quen- 
stedt); zone of Am, Davai, Bassin du Rhone (Dumortier). 
Observations. Quenstedt has confounded two apparently distinct 
species with WN. variabilis, Sow.,—the one under review, which is 
easily differentiated by its inflated form, narrow and flattish anterior 
area, and its deep lunule, and a second (tab. xiii. f. 43), which I refer 
with some doubt to WV. navis, Piette. The N. variabilis, Phillips, is 
referable to V. Hammeri, Goldf. The three species that have thus 
been confounded with WV. variabilis agree in their general configura- 
tion, but are readily to be distinguished from one another and from 
N. variabilis. 
EXPLANATION OF PLATE XXVI. 
. Dentalium limatulum, n. sp., enlarged. 
. Inoceramus incurvatus, n. sp., nat. size. 
. Arca numismalis, n. sp., enlarged. 
Trochus Thetis, Minst. 
. Lxelissa numismalis, Tate, enlarged. 
. Chemnitzia crassicosta, n. sp., enlarged. 
. Cerithium Slatteri, n. sp., enlarged. 
—— ibex, n.sp., enlarged. 
. Pleurotomaria raricostati, n. sp., nat. size; and portion of test, enlarged. 
10. Turbo admirandus, n. sp., enlarged. 
11. Nucula ungulella, n. sp., enlarged. 
12. Leda graphica, n. sp., nat. size. 
13. Anatina numismalis, n. sp., enlarged. 
15. Lima scabricula, n. sp., enlarged. 
17. Littorina biornata, n. sp., enlarged. 
18. Tornatella capricorni, n.sp., enlarged. 
21. Straparolus aratus, n. sp., enlarged. 
bellulus, n.sp., enlarged. 
23. —— Wrightianus, n.sp., enlarged. 
Fig. 
2 OST SP Od be 
Discussion. 
Mr. Boyp Dawxrys had attempted to test these Liassic zones as 
a means of classification of the rocks in Somersetshire, and the result 
had been that he had been unable to accept them as fixing hard and 
fast lines of demarcation ; for he had found three of the distinctive 
Ammonites together in one bed. On our present shores the change of 
one form of molluscan life for another seemed to take place in limited 
areas, and to be dependent on some slight variation of physical con- 
ditions rather than on any widely extended change. There was no 
stratigraphical unconformity between the Middle and Lower Lias in 
many parts of England, whatever might be the case in Gloucestershire, 
Mr. Tarte, in reply, gave an account of the manner in which he 
had arrived at his conclusions, and expressed his assent to the view 
that Ammonite-zones were only of value over limited areas, but con- 
sidered that a triple division in the Lower and a dual division in 
the Middle Lias were well established on paleontological and litho- 
logical features. The break which he had pointed out was palseon- 
tological rather than stratigraphical, though the one might be 
inferred from the other. 
