14 TYPE AMMONITES— IV June 



research proceeds it is seen that certain local names given to Stages, 

 because the strata appeared to be so well developed at the localities, 

 are imperfect for various reasons : the strata are not in true sequence, 

 a middle portion is partially or completely missing ; or the strata are 

 defective at their beginning or their end. Extensions of the geographical 

 term to meet the new discoveries, or to make the name of the Stage 

 correspond with some definite faunal development, are often not con- 

 sidered to be warranted. 



Further, there are complications which increase the difficulties for 

 the memory. Owing to difference in application of a name, differences 

 of local usage and association, the same term for a Stage is found to be 

 applied to deposits of different dates — in England Oxfordian is used 

 for the deposits known as Oxford Clay ; on the Continent it is employed 

 for the later deposits — the Oxford Oolites. A similar result has obtained 

 with the term Portlandian on the Continent : owing to confusion in 

 Ammonite nomenclature, this term came to be used for a large part 

 of what is known in England as Kimmeridgian. 



There are various reasons why zoological names applied to Ages 

 should be more suitable for chronological purposes, and, when once 

 learnt, easier to remember. Chronology is marked by successive faunal 

 developments which, there is reason to think, are world-wide — at any 

 rate, in the case of Ammonites. The difference in Ammonite-fauna 

 between the Mediterranean and the Mid-European provinces, which is 

 supposed to make exact synchronization of some of their respective 

 strata difficult or impossible, is more probably not geographical, but 

 geological — due mainly to differences in the preservation of corresponding 

 strata in the two provinces — in the south is preserved what the north 

 has lost, and vice versa. 



Faunal differences which exist in supposed isochronous strata at 

 localities a few miles apart in the same basin cannot be ascribed to 

 geographical situation making difference of climate. Rightly, therefore, 

 proof is required when faunal differences in two regions are, under 

 similar circumstances, ascribed to geographical causes. Reasonably, in 

 the more southern regions greater abundance of species may be expected ; 

 but a complete disagreement in species between the two regions suggests 

 not geographical, but geological differences — the correlation is at fault, 

 the claim that the strata are truly isochronous may be questioned. 

 When the strata of the Mediterranean and Mid-European provinces are 

 truly isochronous, some community of species in the two regions is to 

 be expected. In earl}^ Jurassic (Liassic) strata such community of 

 species in the two provinces is well enough known ; in late Jurassic 

 (Upper Oolites) strata such community is exceptional. If geographical 

 situation be claimed as the cause in the second case, why was it not a 

 cause in the first ? 



The names given to the different episodes of the faunal succession 

 represent a series of natural phenomena ; therefore they are not arbitrary 

 names — they correspond to what would be the ideal in geographical 

 naming mentioned above. They should express a definite sequence of 

 events — a sequence which in most cases has been proved by repeated 

 research. Therefore, a series of zoological names for Ages, expressing 

 a sequence of zoological facts, should be much easier to remember than 

 any series of geographical names for Stages culled haphazard, as it were. 

 Genus A appeared earlier than genus B, which again preceded genus C — 

 a system of naming which records these facts gives a definite clue to 

 the memory ; the same can only be claimed for geographical names 

 taken from a definite line in one country : it cannot be claimed for them 



