Mrs. McKenny Hughes—Pleistocene Mollusca. 203 
diversion of the river has produced effects which must have been 
common in the case of the uncontrolled rivers of former times. On 
Sheep’s Green there are many small ponds which represent deserted 
portions of the old river-bed; in these many of the less common 
and irregularly distributed freshwater shells occur—abundantly in 
one—rarely, or not at all, in another. For instance, we find Bythinia 
Teachii there, as well as in the ditches further south. In one of the 
ponds seven species of Planorbis, including the two rare forms P. 
nitidus, Mill., and P. fontanus, Light., also Valvata cristata, Mill. 
and many of the Pisidia, are found. 
Mr. Tomlin, who has thoroughly searched that locality, informs 
me that two of the ponds are far richer than any of the others, and 
that he never met with the two rare species of Planorbis, mentioned 
above, elsewhere in this neighbourhood, except odd live specimens 
in some of the other Sheep’s Green ponds. He never found them 
anywhere down the river. 
Along the margin of these pools Carychium minimum, Mill, and 
Hyalina nitida, Mill., abound. Now and then, when the river is 
in flood, the whole Green is under water, and at such times many of 
the shells in these ponds and ditches must be carried away and 
mixed with the common river shells. 
From analogy, therefore, it would appear that the winding about 
of a frequently flooded river, over an alluvial plain, in which ponds 
remained where the deeper parts of the old river-bed had been, 
would most easily account for the difference in the facies of the 
gravel-shells in the different localities. 
A careful study of the distribution and mode of occurrence of the 
gravel fauna and flora ought to give us some clue to the geographical 
changes which have affected the incoming and disappearance of the 
various forms of life. 
The great majority of Mollusca from these gravels are living in 
this district at the present day. A few are locally extinct. Of these 
some are confined to the north and some to the south of England, 
whilst some have disappeared from the British Isles altogether. 
Some of the Mammals, but none of the Mollusca, are totally extinct. 
The shell which seems to indicate the greatest change of con- 
ditions is the Corbicula (Cyrena) fluminalis, Mill. 
It lives at the present time in Sicily,’ in the rivers of Asia Minor 
and Syria and in the Nile. It seems to have made its first appear- 
ance in Britain in the time of the deposition of the Norwich Crag, 
from which it is described and figured by Mr. Searles Wood,” as 
Cyrena consobrina.’ 
It is recorded from the Weybourn Crag and from the Forest Bed. 
In the north of England it is found in the gravels of the Humber, 
of Kelsea Hill, and Hessle; in the basin of the Thames and the 
adjoining district of Essex it has been recorded from Suttonness, 
Clacton-on-Sea, Copford, Greys, Ilford, Erith, and Crayford, Faver- 
1 Geol. Eng. and Wales, H. B. Woodward, 2nd edition, p. 478. 
2 §. V. Wood, ‘‘ The Crag Mollusca,” vol. ii. p. 104, tab. 11, fig, 1. 
3 Tylor, Q.J.G.S. vol. xxv. 1869, p. 66. 
