146 Mr Tawney, On the Upper Bagshot Sands of [Nov. 28, 



After eliminating those which occur both in Barton and M. 

 Headon beds, the per centages become ; common to Barton beds 

 and not to M. Headon 43'4 per cent.; common to M. Headon and 

 not to Barton 21*3 per cent. As far as fossil evidence is concerned 

 therefore, these sands are more related to the Barton beds than 

 they are to the Headon. 



We have next to deal with the argument that they must be 

 classed with the Headon series because both contain C. concavum, 

 Sow. It is true they contain a shell which has hittherto been 

 confounded with that species, but probably falsely so. I made a 

 point of collecting some hundred of this shell, and have little 

 doubt now that it is C. pleurotomoides, Lam.: we wish to correct 

 therefore our former commingling of these species 1 in which we 

 followed previous authors. The object of examining the equiva- 

 lent beds in France was to see whether the distribution of this 

 species there was such an extended one as would have been the 

 case if it ranged from the Long Mead End sands to the top of the 

 M. Headon. I am much indebted to Prof. Hebert and M. Munier- 

 Chalmas, the eminent authorities at the Sorbonne, for pointing 

 out to me that the Mortefontaine sands do not contain G. concavum 

 at all, and that the shell so common at that horizon is G. pleuroto- 

 moides, Lam., which they deem distinct. The French sands near 

 Mortefontaine preserve the shells in most perfect condition, so that 

 there is no doubt about the identification there. Unfortunately 

 at Long Mead End, the shells are so corroded and the ornaments 

 so poorly preserved that the distinctness from the M. Headon 

 shell has not been hitherto recognised. On comparing it however 

 on the one hand with examples brought from near Mortefontaine, 

 and on the other hand with the M. Headon shell, it becomes 

 evident that the Long Mead End species agrees with the former 

 only 2 . It differs from G. concavum not only in the tubercles 

 when preserved, but also in the curve of the mouth aperture, and 

 form of the notch or sinus there. 



Owing to the courtesy of M. Munier-Chalmas, I have been 

 made acquainted with results which he is about to publish, and 

 which point to a much greater parallelism between the French 

 and English series than has hitherto been acknowledged. The 

 Mortefontaine sands are merely the upper part of the Sables de 



1 Q.J.G.S. xxxvu. p. 107. 



8 So long as the Cerithiuin of both horizons, viz. the Long Mead End sands and 

 Middle Headon was determined as identical, in that sense C. concavum had a wide 

 range in England. We think it, however, due to a misconception to state as has 

 been done [Q. J. G. S. xxxvu. p. 126] that all foreign geologists regard the zone of 

 this species as the top of the Barton. They have written only of the Long Mead 

 End or Mortefontaine sands as such. We may trust, I think, foreign geologists 

 to know the relations between the Mortefontaine sands and the St Ouen lime- 

 stone. 



