50 TYPE AMMONITES— III Sept. 



Pleurophorites : Am. hdlatus ; Ouen., lxxvii, 9, is near to Sphcero- 

 morphites, but differs as already mentioned. 



Chronological Sequence : For the English Tulitidae this is 

 important. There are the following data : in the Fullers' Earth Rock 

 of Dorset-Somerset brown (ironshot) limestones (Milborne Beds) ; 

 are later than the whitish, chalky limestones (Thornford Beds) ; in 

 the Great Oolite of Gloucestershire shelly, oolitic beds are later than 

 whitish, non-oolitic beds. Thus the genera of Tulitidse occupy two 

 distinct horizons. But more analysis is possible, by observation of 

 smaller differences in the matrices of the various specimens. 



Certain technical terms may facilitate this and future enquiries. 

 For instance, the Thornford Beds, the Milborne Beds and the Minchin- 

 hampton Beds are all similar in character, in that they are limestones : 

 therefore they maybe said to be ho moeo petrous ; but they are 

 distinct from argillaceous or arenaceous strata : the Lower Fullers' 

 Earth Clay and the Fullers' Earth Rock are heteropetrous. But 

 the Milborne Beds (ironshot), the Thornford Beds (chalky, not oolitic) 

 and the Minchinhampton Beds (the upper part, oolitic and shelly) differ 

 from one another in a minor degree : they are heterolithous ; but 

 any one of these deposits shows still further differences : the Thornford 

 Beds are not sequentially homolithous, — the matrices of the Tulitids 

 show appreciable differences, though these differences have not been 

 sufficient to induce any observer to make a detailed section of the beds 

 exposed in Troll quarry — some 5 feet : the need for such detail was not 

 understood. But, as these beds show such differences, they must be 

 classed as homoeolithous ; not as homolithous ; while the differences 

 are presumed to indicate differences in time of deposition — a chrono- 

 logical sequence : such surmise is justifiable, because the heterolithous 

 differences between the Thornford and the Milborne Beds indicate a 

 known sequence. The sequence within the Thornford Beds must at 

 present be a matter of surmise. Knowledge may be gained by noting 

 which are isochronous deposits of different areas — those beds which are 

 isidophorus, yielding species equal in value (Ja-os. equal, (l8os, form, 

 or species, <pop<'is, bearing), being regarded as isochronous. The yielding 

 of the same species is not insisted upon ; but beds in two areas which 

 yield species of, say, Tulites are reckoned as isochronous, until there 

 be evidence to the contrary. By a parity of reasoning beds which do 

 not fulfil such condition, beds which are anisidophorus, are, 

 presumably, anisochronous, not equal in date. These are the arguments 

 of similar and dissimilar faunas carried out on limited groups of organisms, 

 like, for instance, Ammonites : therefore words compounded of species 

 («8os) are considered as more desirable than any compounded with 

 fauna : the latter should have a general application. For arguments 

 and methods of work with regard to the phenomena of similar (limited) 

 faunas (isidophory) and dissimilar faunas (anisidophory) the author's 

 papers on Jurassic Chronology (Quart. Journ. Geol. Soc, LXXIII, 

 1918, 257 ; LXXVI, 1920, 62) may be consulted. 



Geological Sequence : Working on the methods discussed above, 

 the following geological sequences may be given- — as regards major 

 details the succession is known, but as regards minor details it is a case 

 of surmise and inference. However, from what has been said above, 

 and from what is stated in Tables IV, V, the observations upon which 

 the work is based, and the manner in which evidence is used, may be 

 understood. 



