50 TYPE AMMONITES— IV Aug. 



Minchinhampton, possibly only the upper part of that (Oxyceritan, 

 suspensus, T.A. CCCXLVI). This zone is spoken of as " les premiers 

 sediments Bathoniens " (Lissajous, p. i6), reposing, in the neighbourhood 

 of Macon, upon strata of the " Bajocien," presumably Parkinsonian, 

 Garantiana. Such superposition has led the author to think that the 

 time-interval between the deposits is a small one ; but in England, 

 between the Inferior-Oolite strata from which my species came 

 (Zigzagiceratan, zigzag) and the Great Oolite strata of Minchinhampton, 

 there are the following thick deposits : Fullers' Earth, Stonesfield Slate 

 and much of the Great Oolite strata of Bath, which are earlier than 

 the beds of Minchinhampton. Separating the two deposits there may 

 be a thickness of as much as 500 feet. 



To say that the same three species of Ammonites endured through 

 a time-interval represented by the deposition of some 500 feet — that 

 the species which belong to the zigzag bed of the Inferior Oolite of 

 England migrated to re-appear after all that interval in strata of the 

 east of France, equivalent to the Great-Oolite beds of Minchinhampton, 

 is to ask for something quite contrary to all experience in regard to the 

 duration of Ammonite species. If they are the same species, the explana- 

 tion in the Macon case might more reasonably be derivation from 

 destroyed local deposits of zigzag date. But in my opinion they are not 

 the same species — there is not even sufficient external likeness to warrant 

 the assumption ; while, had the suture-lines been investigated, I feel 

 certain that undoubted differences would have been revealed. 



From this may be learnt two rather important lessons : i, the 

 danger of claiming identity for species of quite different dates ; 2, that 

 it is as important to prove identity as to disprove it — that external 

 similarity is an untrustworthy guide, and that it is necessary to prove 

 identity of suture-line before claiming similar-looking forms as the same 

 species. More trouble is caused by placing different forms under the 

 same name than by putting the same forms under different names. 



Another warning seems advisable — that it is dangerous to place a 

 series of, say, Continental forms as varieties of an English species without 

 having among them an example which is identical with the English type. 

 This should be the first thing to establish, and should be the starting- 

 point of the investigation. The converse is my method in the identification 

 of English with Continental species. This instance may be given : it is 

 dangerous to regard, say. Continental Stepheocerates as varieties of 

 Stepheoceras humphriesianum until an exact Continental counterpart of 

 that species can be produced. For this reason — the stratum yielding 

 S. humphriesianum is preserved only very locally in this country, and it 

 was, where preserved, nearly removed by penecontemporaneous erosion — 

 the half condition of the type is due to the planing away which it suffered 

 while lying in the rock. It has yet to be proved that any stratum of 

 exact date with that of 5. humphriesianum exists on the Continent : 

 it may have been altogether destroyed from there. And if the Continental 

 forms of humphriesiamim-SiSpect are not exactly synchronous with the 

 English species, they cannot be varieties of it. Stepheoceratids persisted 

 through several hemerae, but there is not yet evidence that Stepheoceras 

 did. 



The Chronological Analysis of genera in Vols. Ill, IV, given in 

 p. 48, reveals a very large number of generic names. But wholly false 

 conclusions may be drawn from this. One reason for the number is 

 that an attempt is being made to give a synopsis of the rich Jurassic 

 Ammonite fauna, so that, at least, there may be a generic name to give 



