94 Correspondence — Clinton G. E. Daickins. 



index-fossil of a zone must be distributed "all through its zone," 

 and what he means by this expression. I am sure that he does not 

 expect to find the chosen fossil in every foot of the chalk which 

 makes up the zone. I suppose, therefore, he means that it ought to 

 occur at frequent intervals throughout the zone, and that the total 

 thickness of bods in which it does occur should be greater than that 

 of those in which it does not. 



I greatly wish that fossils would occur in such a well-regulated 

 manner, but unfortunately their behaviour often falls sadly short of 

 what we should like it to be. Mi*. Br3'done must surely have 

 forgotten that Marsupites is not a common fossil throughout the 

 zone of which it is accepted as the index. In fact, it is common 

 only in the Marsupites band or subzone, and is rare or absent in 

 the Uintacrinus band. Yet I am not aware that anyone has objected 

 to its being used as the index-fossil of the zone, and I sincerely hope 

 that no such objection will ever be taken. 



Again, has Mr. Brydone considered the case of the zone of Act. 

 quadratus, where that species (as now restricted) only occurs rarely, 

 especially in the higher part of the zone. It is true that Mr. Eowe 

 has proposed to take Offaster pilula as the index-fossil, because it 

 is common throughout, but this generally occurs at intervals only, 

 being common in spots or in bands and rare or absent in the inter- 

 mediate beds, just as Ostrea hinata seems to be absent from certain 

 beds in the Trimmingham Chalk. 



From the succession of beds given by Mr. Brydone on p. 14 of 

 this Magazine, and assuming his group 3 to be identical with part 

 of his group 4, it is seen that 0. hinata occurs abundantly at four 

 horizons in the series, and that it occurs in all three divisions. This 

 is quite sufficient to satisfy all reasonable demands on any fossil for 

 qualification as the index of a zone ; consequently I must maintain 

 the propriety of my choice, and must object to any other species 

 being substituted for Ostrea hinata, unless a much better reason can 

 be given than that advanced by Mi\ Brydone. 



A. J. Jukes-Browne. 



Floriston, Torquay. 



DISCOVERY OF EXOGYEA SINUATA IN THE LOWEE GEEENSAND 

 OF CULHAM, NEAE OXFOED. 



Sir. — It might interest your readers to hear of the finding of 

 a specimen of Exocjyra sinnata by Mr. W. D. Hutchinson and myself 

 in the Lower Greensand of Culhara, near Oxford. 



The specimen is a large one. and was found in a bed of coarse 

 laminated sandstone, in a neighbourhood where the Greensand has 

 been considered unfossiliferous. Clinton G. E. Dawkins 



(Balliol College, Oxford). 



6, Larkstone Terrace, Ilfracombe. 



