248 MESSES. G. W. LAMPLUGH AND J. F. WALKER ON [May I9O3, 



existence in this place, and they disappeared ; but were perpetuated 

 in other places where the conditions were more suitable, and re- 

 established themselves when the circumstances again became fitting. 

 From the richness of the fauna both in species and individuals, it is 

 unlikely that even at Shenley have we reached the lowest range of 

 the so-called Upper Cretaceous forms; although, if it were 

 possible to trace the fauna continuously downward, we should, of 

 course, expect to find an increasing preponderance hf exclusively 

 Lower Cretaceous species. 



The Lower Greensand of Central and Southern England, con- 

 sidered as a whole, forms a well-defined stratigraphical unit which 

 has been classified as Lower Cretaceous ever since this division was 

 established ; and it seems to us to be undesirable to break up this 

 classification, unless far stronger reasons are forthcoming than can 

 be adduced in the present case. Setting other considerations aside, 

 since the Shenley fossil-bed is lithologically part of the Lower 

 Greensand, its fauna is entitled to be classed as Lower Cretaceous 

 by priority of nomenclature, failing weightier cause for separating 

 it therefrom. 



Finally, as regards the palseontologieal evidence, it is certain that 

 the prevalent conception in this country of what constitutes a Lower 

 Cretaceous fauna will require considerable modification. Because 

 it happens that the closing stages of the Lower Cretaceous Period 

 in the South and South-East of England are represented mainly 

 by unfossiliferous sands ; and that the l initial stages of the same 

 period are represented in the same region by beds of freshwater 

 origin : it has come to pass that our ideas concerning the marine 

 fauna proper to the Lower Cretaceous have been based upon the 

 fauna of only a very limited portion of the period. Consequently 

 we find that a large number of the fossils of the ' coprolite-beds ' 

 at or near the base of the Lower Greensand have been rejected 

 as ' derivatives,' on account of their Jurassic aspect or mineral 

 condition ; while as regards the uppermost beds, forming the zone 

 of Ammonites mammillatus, because these have yielded fossils that 

 show Upper Cretaceous affinities, it is claimed that they cannot be 

 retained in the Lower Cretaceous but must be classed with the 

 Gault. Too often do we lose sight of the essentially-arbitrary 

 character of the boundaries adopted for our stratigraphical divisions, 

 which are for the most part based upon local conditions that can 

 only have persisted over areas of limited extent. And it is certain 

 that the more fully we become acquainted with the sequence of 

 events, the more difficult will it be, either from the stratigraphical 

 or from the palaeontological standpoint, to maintain our preconceived 

 ideas as to the methods to be adopted in determining the limits of 

 our systems. If our present Lower Cretaceous system is to be 

 maintained, we shall have to recognize towards its base certain 

 life-forms as Lower Cretaceous which have been hitherto regarded 

 as exclusively Jurassic, and toward its summit other life-forms as 

 Lower Cretaceous which were originally supposed to belong exclu- 

 sively to the Upper Cretaceous. Difficulties of similar character 



