WEALDEN FOSSILS. 
409 
great river flowing from the north-east. The Weald clay is of a pale 
grey and is near 1,000 feet thick and contains hands of shelly lime- 
stone, known as Petworth marble, which are thickly studded with 
fossils of Vivipara, chiefly Vivipara fluviorum Sowerby. The fossils 
are distinctly freshwater species and embrace Vivipara, TJnio, Mela- 
nopsis, Corbicula, etc. 
In North America, where more favourable conditions existed at this 
period for the preservation of fossil remains, many subgeneric groups 
flourishing at the present day are found to have lived as early as the 
close of the cretaceous epoch. Acella, LimnojViysa, Gyraulus, Bathy- 
omphalus, Aplecta, Physa, and other subgenera of Limnceidw are 
distinctly represented in the Laramie and other strata. The Helicidce 
were almost as diversified as at the present day, forms apparently 
referable to Aglaia, Epiphragmophora, Strobila, Pyramidula, Trio- 
dopsis, etc., being found, while the Unionidw showed also a remarkable 
differentiation, as subordinate types largely identical with many still 
Mississippian, even then existed, though many living tj^pes had appa- 
rently no living representatives at that period. 
The known British fossils of this era are : 
MELAXIID.E. 
Melania popei (J. de C. Sow.), 
Melanopsis attenuata J. de C. Sow., 
popei J. de C. Sow., 
tricarinata J. de C. Sow. 
VIVIPAEID.E. 
Viripara carinifera (J. de C. Sow.), 
elonqata (J. de C. Sow.), 
fluviorum (J. de C. Sow.), 
sussexiensis (Mantell). 
NERITIU.E. 
Neritina flttoni Mantell. 
UNIOXID.E. 
Unio adtoicus J. de C. Sow., 
antiquus J. de C. Sow., 
comqjressus J. de C. Sow., 
UNIONID.E. 
Unio cordiformis J. de C. Sow., 
gualtieri J. de C. Sow., 
mantcUi J. de C. Sow., 
martini J. de C. Sow., 
porrectus J. de C. Sow., 
snbfruncafus J. de C. Sow., 
valdensis Mantell. 
CORBICULID.^:. 
Corhicida nngxdata (J. de C. Sow.), 
elongata (J. de C. Sow.), 
gibhosa fJ. de C. Sow.), 
major (J. de C. Sow.), 
media (J. de C. Sow.), 
membranacea (.1. de C. Sow.), 
subquadrata (J. de C. Sow.). 
The Tertiary period opens with the elevation of the gi’eater part 
of the British Isles above the sea, which were therefore subjected to 
the unavoidable denudation and waste of surface that would furnish 
the material for the formation of rocks elsewhere, instead of being 
more or less submerged and receiving deposits from the waste of 
other lands ; an interval thus exists between the lower tertiary beds 
and the cretaceous strata, and the lapse of time this gap represents 
mu.st have been enormous, as with perhaps one or two exceptions all 
the cretaceous species have disappeared, before the deposition of the 
lowest tertiary beds, and this is the more remarkable as the number 
