Inverted Mass of Upper Cretaceous Strata. 5 



Walker, whose reading of the facts implied an anomaly so arresting 

 as to make further investigation desirable. Nevertheless, those 

 authors were more impressed by the stratigraphical evidence, as 

 understood by them, and remained convinced that the bed occupied 

 a normal position below the Gault. They met the difficulty by 

 explanations which we have always considered to be inadequate,'^ 

 while they treated as so-called " varieties " some of the species 

 which appeared to occur so far below their usual horizon. Arguing 

 from stratigraphical inferences, they claimed to demonstrate that 

 these species in reality made a much earlier appearance and possessed 

 a much longer vertical range than had 23reviously been suspected. 



We need make no lengthy comment here on this manner of 

 dealing with the palseontological aspects of the bed ; but we desire 

 to record that from the first we have entirely disagreed with the 

 interpretation given. It has always appeared clear to us that if the 

 bed contains both derived and indigenous fossils its geological age 

 must be that of its latest species. To assume such an early 

 appearance and long persistence for the particular assemblage of 

 brachiopods and echinoids mentioned, has always seemed to us to 

 be making an arbitrary use of the fossil-evidence. Some biological 

 considerations which are in conflict with the assumption need not 

 be entered into here. The zonal significance of these fossils had been 

 firmly established by the well-known facts of their restricted 

 occurrence both in this country and on the Continent. Hence it 

 appeared a hazardous proceeding to ignore the indications they 

 afforded and to place more reliance on stratigraphical appearances. 



In subsequent years the above-mentioned authors collected 

 additional fossils from the limestone-lenticles with brachiopods, 

 resulting in further support for our opinion that the true position 

 of the bed is at the base of the Cenomanian. Mr. H. Woods has 

 kindly communicated to us the names of some species represented 

 in the collection obtained by Mr. Walker, now preserved in the 

 Sedgwick Museum, Cambridge. They include the brachyurous 

 crustacean Cyphonotiis incertus Bell, first described from the so- 

 called " Upper Greensand " of Horningsham, Wiltshire. The type 

 probably came from some exposure in the basal part of the 

 Cenomanian, at which horizon the species is found at other localities 

 in Wiltshire and in Somerset. This fossil also occurs in the 

 Cambridge Greensand. Mr. Woods points out to us that no earlier 

 species of this genus is known. The lamellibranchs at Cambridge 

 include Pecten (Camptonectes) eurvatus Geinitz, which occurs in the 

 Upper Greensand of Great Haldon and in the Chloritic Marl, and 

 Cypr.imeria (Cijclorisma) rotomagensis (d'Orb.), a species of the 

 basal Cenomanian of Wiltshire. Among the lamellibranchs found 

 in the limestone by Mr. Lamplugh and presented by him to the 

 Geological Survey occurs Isoarca obesa (d'Orb.), which is not known 

 elsewhere from below the Chloritic Marl. We have ourselves collected 

 1 Op. cit., pp. 247-9. 



