1922 CHROXOLOGY 15 



when they are taken, as they necessarily must be, from various countries. 

 There is nothing to guide the memory as to whether a name taken from 

 a place in England preceded or succeeded one taken from a place in 

 France or in Germany. 



Table I, presented in pp. 6 — 13, illustrates these remarks. It is 

 to be followed by another Table setting forth the sequences of hemerae 

 which make up the different Ages, together ^^ith such stratal correlations 

 as the present, admittedly very incomplete state of knowledge allows. 

 Systematic investigation of hemeral sequence is only just beginning : 

 it is hampered by lack of names — the practice of apph'ing to homoeo- 

 morphous species at widely different horizons a designation which may, 

 at the best, belong perhaps rightly only to one of them, is responsible 

 for much trouble. The others may all lack names and from such lack 

 are difficult to record with precision. All this has to be allowed for 

 in considering the Tables. 



The history and evolution of these Tables, so far as regards the 

 Lias and the Lower and Middle Oolites, may be found in this work : 

 I, p. xvi, II, p. X, III, pp. 9, 10, 40, 51, and in the Author's papers, 

 Q.J.G.S., LXVI, 1910, pp. 52-108, LXXIII, pp. 257-327 ; LXXVI. 

 pp. 62-103 • among these \nll be found references to other papers, to 

 the work of other authors, and to the labours of many kind helpers. 

 Dr. W. D. Lang, Dr. L. F. Spath, Dr. A. E. Trueman^ Mr. J. Pringle 

 and Mr. A. Templeman have also been carrpng the work further in the 

 Lias, and are thanked for all their kind information. 



As regards the Upper Oolites, the Author is greatly indebted to the 

 masterly works of Dr. Hans Salfeld — particularly to his Gliederung 

 d. oberen Jura in Nordwesteuropa ; X. Jahrb. 1913, Beil.-Bd. XXX\TI, 

 pp. 125-246. He also acknowledges with thanks much kind help and 

 information from Dr. A. ^lorley Da\ies, Dr. F. L. Kitchin, Mr. J. Pringle, 

 Mr. C. P. Chatwin, and from many others who have aided b}' kindly 

 submitting specimens. * 



^ome explanation of Table I may be given. No claim is made 

 that all the Ages mentioned in the Table should be regarded as belonging 

 to the Jurassic Period — some of early date may be claimed for the 

 Triassic, and some of late date for Cretaceous — or some faunas now 

 regarded as Cretaceous may be found to have greater affinity with 

 Jurassic. 



The heterogeneous terms — Cretaceous, Jurassic, Triassic — are unsuit- 

 able for chronology. That demands some such dixision as 



Baculitoidic Period (Cretaceous) 

 Ammonitoidic Period (Jurassic) 

 Ceratitoidic Period (Triassic) 



where BacuUtoid ma}' be taken to express not only Baculites, but the 

 uncoiled or aberrantly-coiled species in general, which are so characteristic 

 of the third Period of the Mesozoic. 



Di\4sion of the Ammonitoidic Period into Epochs will be required. 

 The family or super-family names of Ammonites seem unsuitable — 

 too limited in the first case, too comprehensive in the second. Rather, 

 what have to be expressed are the morphological phases of Ammonite 

 development which are dominant at certain times, as, for instance, that 

 towards the later part of the Jurassic (Ammonitoidic) Period planulate 

 Ammonites are the dominant feature— successive waves of heterogenetic 

 homceomorphs which have arrived at the planulate condition along 

 manv different fines. 



