Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/574

 Dimensions. Length 0.35 inch ; breadth 0.25 inch ; thickness 0.25 inch.

Geological position. Zone of Ammonites Jamesoni: Cheltenham (R. T.) ; zone of Am. margaritatus, Hinterweiler, Swabia (Quenstedt) ; zone of Am. Davoei, Bassin du Rhone (Dumortier).

Observations. Quenstedt has confounded two apparently distinct species with N. 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 N. navis, Piette. The N. variabilis, Phillips, is referable to N. Hammeri, Goldf. The three species that have thus been confounded with N. variabilis agree in their general configuration, but are readily to be distinguished from one another and from N. variabilis.

EXPLANATION OF PLATE XXVI.

Pig. 1. Dentalium limatulum, n. sp., enlarged.

2. Inoceramus incurvatus, n. sp., nat. size.

3. Arca numismalis, n. sp., enlarged.

4. Trochus Thetis, Munst.

5. Exelissa numismalis, Tate, enlarged.

6. Chemnitzia crassicosta, n. sp., enlarged.

7. Cerithium Slatteri, n. sp., enlarged.

8. ibex, n. sp., enlarged.

9. 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.

22. — bellulus, n. sp., enlarged.

23. — Wrightianus, n. sp., enlarged.

Discussion.

Mr. Boyd Dawkins 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 conditions 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. Tate, 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 considered that a triple division in the Lower and a' dual division in the Middle Lias were well established on palseontological and lithological features. The break which he had pointed out was palaeontological rather than stratigraphical, though the one might be inferred from the other.