STRATA OF THE HAMPSHIRE BASIN. 149 



varied marine fauna, which, as I shall hereafter show, characterizes 

 a higher horizon in the geological series as displayed upon the 

 Continent than is represented by the brackish-water fauna of Headon 

 Hill and Hordwell Cliff. 



Now, although in such series of strata as those of the Isle of 

 Wight we might be prepared to find marine beds passing at certain 

 points into others exhibiting evidence of brackish-water conditions, 

 yet it is impossible to believe that a thick mass of marine strata, 

 maintaining such a uniformity of character and of fossils at points 

 so distant as Whitecliff Bay, Colwell Bay, and Brockenhurst, could 

 lose all their distinctive characters and pass into brackish -water 

 strata at intermediate points like Headon Hill and Hordwell Cliff. 

 Moreover, as I shall proceed to show, the marine fossils of these two 

 sets of beds are not identical in character, as has been supposed, but 

 very distinct, the faunas being such as, at many points upon the 

 Continent, characterize two perfectly distinct horizons. 



This distinction between the fauna of the Colwell-Bay beds and 

 that of the Headon-Hill and Hordwell-Cliff beds is rendered stri- 

 kingly apparent if we direct our attention to the species and varieties 

 by which certain genera are represented in these two deposits. The 

 genus Cerithium especially affords very interesting and valuable 

 evidence bearing on this point ; and it was, indeed, while engaged 

 in a series of researches, commenced many years ago, with a view 

 to the determination of the genealogy and lines of descent of 

 the forms of this group, that I first detected the serious errors 

 which have crept into our classification and correlation of the 

 strata we are now considering. While the form now known as 

 Cerithium pseudocinctum, d'Orb. (with its variety C. tnzonatum, 

 Morris) occurs in great abundance both in the marine beds of Col- 

 well Bay and all through the Headon series, two well-marked forms, 

 Cerithium ventricosum, Sow., and C. concavum, Sow. (with its varie- 

 ties C. pleurotomoides, Lam., and C. rusticum, Desh.), are entirely 

 confined to the Headon beds, occurring similarly at Hordwell Cliff. 

 At both these latter localities these two forms of Cerithiinn are 

 found in such prodigious abundance as to constitute the most cha- 

 racteristic fossils of the beds ; and their total absence from the 

 Whitecliff-Bay, the Colwell-Bay, and the jSTew-Porest beds is a most 

 significant circumstance. 



Xow, as long ago shown by M. Hebert, the form of Cerithium 

 known in this country as C. concavum, Sow., and in France as C. 

 rusticum, Desh., and C. jpileurotomoides, Lam., is found at a very 

 definite horizon in the Paris basin — " the Upper fossilifcrous zone 

 of Mortefontaine and Monnevillc &c." — the beds there, like those 

 of Headon Hill, being characterized by the extreme abundance of 

 that fossil. More recently Dr. C. Mayer, of Zurich, whose researches 

 have contributed greatly to our knowledge of the exact correlation of 

 the various Tertiary deposits, has insisted upon the importance of 

 this palaeontological horizon, which he has distinguished by the name 

 of " the Zone of Cerithium concavum''^. Dr. Sandberger, too, fully 

 * C. Mayer, ' Table cles Terrains Tertiaires Inftrieurs,' Zurich, 1875. 



