Reports & Proceedings — Geological Society of London. 333 



the other two, subdivisions seem necessary, and each is apportioned 

 into three Ages, as follows: — 



Proposed Navies. Old Terms. 



Hiviccian. ^1 



Wessexian. - Charmouthian. 



Raasayan. J 



Deiran. \ 



Mercian. J- Sinemurian. 



Lymian. J 



These, with the Domerian, each contain on an average about ten 

 hemerae, the grouping being controlled by the dominance of ammonite 

 families of phases thereof — thus, Domerian: Age of Amaltheids; 

 llaasayan : Age of Deroceratidas and Echioceratidse. It is obvious 

 that, with this increase in the number of hemerae, the number of 

 local non-sequences is greatly increased. Some comparative diagrams 

 illustrate this. 



One of the most interesting discoveries which has resulted, partly 

 from the great thickness of Scottish strata investigated and collected 

 from, partly from comparisons with other areas, is that the so-called 

 li armatum Zone" of the English Midlands and that of the Radstock 

 district, of Yorkshire and of the Scottish Isles, are not isochronous, 

 but are separated by a time-interval which corresponds to a thickness 

 of some 300 feet of deposit in the Scottish area. Thus, instead of 

 the simple descending sequence 



Deroceras armatum, 

 Echioceras raricostatum, 



there is this sequence ascertained : 



An upper Deroceras horizon, 



An upper Echioceras horizon in three distinct stages, 



A lower Deroceras horizon, 



A lower Echioceras horizon with some Armatoids ; 



and even now possibly this is not the end of the complication. This 

 alternation of Ueroceras and Echioceras involves a phenomenon which 

 the author calls " faunal repetition ", and it is a reasonable supposition 

 that this is not a solitary case — that is to say, doubt is at once thrown 

 on the contemporaneity of other so-called " zones" where they have 

 been determined in different areas by the presence of certain species 

 of a genus — the species admittedly not the same — or hj the alleged 

 presence of a single species on specific determination insufficiently 

 rigid. . The cases of zones determined on the lucus a non lucen&o 

 principle — the strata in correct intermediate position, but with the 

 index zonal species conspicuously absent — seem especially to invite 

 scepticism. 



Three appendices are given — one, palseontological, containing 

 descriptions of certain notable species, mostly new; another, 

 historical, containing notes on certain ammonites described and 

 figured by Wright in a paper published some years prior to the issue 

 of his monograph : it affords clues to the interpretation of his 

 species, to the recognition of some of his missing types, to the 

 identity of certain figures in Keynes's monograph, and to the geo- 

 graphical distribution of species — a matter of particular importance 



