UPPER EOCENE (bARTON AND UPPER BAGSHOT FORMATIONS). 591 



the Chamas, an assemblage of splendidly preserved bivalves com- 

 prises many species of Tellina, Lucincc, Axinus, Anomia, Cardium 

 ( porulosum), Panopcea, Solen, Terebratula, &c., most having the 

 valves united, as if they had died in situ. On the other hand, 

 nearly all the vast array of Barton species of Pleurotoma and Sca- 

 laria^ most of the Fusi, together with all the species of Ficula, 

 Terehra^ Triton, Littorina, Cassidaria, and many others are absent 

 from it. Other species become scarce and stunted, such as Murecc 

 aspei\ Typhis pungens, Fusus porrectus ; others, again, like Valuta 

 Imtatrix, are represented by modified forms, whilst others, like 

 Murex minax and Voluta atJdeta, seem to be derived and water- 

 worn. It contains, moreover, hardly any of the minute species 

 which so abound in the lower beds, except some of the Bullae and 

 their allies. The fauna contains altogether 170 known species, and 

 differs, as a whole, more from that of the Lower and even Middle 

 Barton than does that of the Lower Barton from the Upper Brackle- 

 sham. The change is, indeed, far greater than appears in the 

 tabulated list ; for though stray specimens of so many species lingered 

 on or are derived, and therefore occur in the column of Upper Barton 

 fossils, practically the entire fauna, except some bivalves, is different. 

 The bulk of what may be considered the typical Barton species, in- 

 cluding such forms as Fusus longcevus, Rostellaria ampla, Voluta 

 luctatrix, V. amhigua, V. atlileta^ Murex minax, Cassidaria nodosa, 

 &c., have disappeared. It is difficult to say positively whether the 

 change was from deeper to shallower water or the reverse ; but the 

 shell-bed preceding the Chama-hed plainly indicates a long period 

 when no mud was being deposited, and the Chama-bed itself was 

 formed in clear water. The fact of the bed being crowded through- 

 out with full-grown, thick-shelled Mollusca, is corroborative evidence 

 that it was formed very slowly. The river, with its turbid water, 

 must therefore have been diverted, and the previous fauna, fitted to 

 rest on an oozy bottom, suddenly gave place to one requiring bright 

 water and cleaner sand. The Chania and Cowry are preeminently 

 such ; and the substitution of fresh species of Voluta, Typhis, Murex, 

 &c. for those preexisting may be taken to mean an immigration of 

 species consequent on this change, rather than evolutionary progress. 

 The bed is altogether as remarkable as any in the Eocene, and cuts 

 into the series as unexpectedly as the Lower Bracklesham Beds, or 

 the coral zone at Brockenhurst. It shows, like so many other 

 abrupt transitions in the Eocene, that a relatively slight change in 

 physical conditions makes a far greater impression on the succession 

 of life in a formation, than would be occasioned by an enormous 

 lapse of time without such a change. 



The Becton-Bunny Beds mark another change, so abrupt that 

 opinions have been divided as to whether the Barton series should 

 end here, excluding these and the overlying sands, hitherto called 

 Upper Bagshot in Hampshire. Prestwich * agreed with Wright, 

 and included the latter in the Barton Series, because they are highly 



* Quart. Journ. GeoL Soc.vol. xiii. (1857) p. 109. 

 Q.J.G.S. No. 175. 2r 



