3)art 4] JURASSIC chroxologt : lias. 275 



'There are two main differences : one, his placing of turneri, upon 

 which I have abeady commented (p. 271) ; the other, his placing, 

 on the evidence of local deposits, a scipionianum zone between 

 Agassiceras {saiizeanuiii) and gmuendense ; this I am quite pre- 

 pared to accept. The omission of the verciiigetorix horizon was 

 expected : the area under Mr. Tutcher's observation lacks the 

 fauna, so far as my knowledge goes. 



With regard to our use of the term turneri zone, it has been 

 objected that this term should be employed in the sense and for 

 the strata to which it was originally applied by Wright. This 

 dictum I traverse. I would argue that the zone of turneri is 

 determined not b}^ the author, but by the fossil. The zone of 

 turneri is the deposit made while A. turneri was in existence : 

 it is the deposit of the hemera turneri. If a writer misidentifies 

 A. turneri, he misnames its deposit turneri zone. If a future 

 historian identified Queen Elizabeth as Queen Victoria, and called 

 the Elizabethan Period the Victorian Age, he would not thereby 

 make it so and shackle future writers. The Victorian Age was 

 determined by the time when Queen Victoria lived ; and the turneri 

 hemera, and the deposit made then — the zone — must likewise be 

 determined by the time when A. turneri lived. 



As to wlao is right or ^wong in the determination of 

 Ammonites turneri I am not yet prepared to argue. 



VI. Summary. 



The various horizons of the Ages dealt with in this paper may 

 now be brought together in sequence ; and a table (Table IX, 

 pp. 276-77) may be given of certain notable areas of Grreat Britain 

 and the Continent, recording the presence of the faunal horizons 

 in Scotland, the North and South of England, the Rhone Basin 

 (Dumortier), and Wiirtemberg (Swabia, Quenstedt). The results 

 are interesting, and they suggest that a more extended analysis on 

 these lines might yield some rather striking information. Such 

 ^analysis may, perhaps, be attempted at some later date. 



VII. Conclusio:n-. 



This is a sketch, much of it theoretical. Its main purpose is to 

 be a basis for further work, indicating the details and evidence 

 which investigators should look for. Much research is yet neces- 

 sary to obtain perhaps even an accurate sequence ; for it is not 

 claimed that all the sequences are proved : they are offered as 

 interpretations of the present available evidence. 



One lesson which may, perhaps, be learnt from this study is that 

 great indeed as is the advantage of collecting specimens in situ, 

 yet it is not absolutely essential : in some cases, owing to paucity 

 of sediment and to derivation of specimens, it may even lead to 

 erroneous conclusions. As collecting in situ is very often quite 

 impracticable, it may be well to remember that much can be done 

 without it, provided specimens are properly localized. For the 



z2 



