268 
ELEMENTS OF GEOLOGY. 
same relative position. In these beds at Bromley, Dr. Buck- 
land found a large pebble to which five full-grown oysters 
w^ere affixed, in such a manner as to show that they had 
commenced their first growth upon it, and remained attach¬ 
ed to it through life. 
In several places, as at Woolwich on the Thames, at New- 
haven in Sussex, and elsewhere, a mixture of marine and 
fresh-water testacea distinguishes this member of the series. 
Among the latter, Cyrena cuneiformis (see Fig. 216) and Me- 
lania mqiiinata (see Fig. 217) are very common, as in beds 
Fig. 216. Fig. 21T. 
Cyrena cuneiformis, Sow. Natural size. Melania {Melanatria) m- 
Woolwich clays. qninata, Des. Syn. 
Cerithium melanoides. 
Sow. Woolwich clays. 
of corresponding age in France. They clearly indicate 
points where rivers entered the Eocene sea. Usually there 
is a mixture of brackish, fresh-water, and marine shells, and 
sometimes, as at Woolwich, proofs of the river and the sea 
having successively prevailed on the same spot. At New 
Charlton, in the suburbs of Woolwich, Mr. de la Condamine 
discovered in 1849, and pointed out to me, a la^^er of sand 
associated with well-rounded flint pebbles in which numer¬ 
ous individuals of the Cyrena tellinella were seen standing 
endwise with both their valves united, the siphonal extrem¬ 
ity of each shell being uppermost, as would happen if the 
mollusks had died in their natural position. I have de¬ 
scribed* a bank of sandy mud, in the delta of the Alabama 
River at Mobile, on the borders of the Gulf of Mexico, where 
in 1846 I dug out at low tide specimens of living species of 
Cyrena and of a Gnathodo7i^ which were similarly placed 
with their shells erect, or in a posture which enables the an¬ 
imal to protrude its siphon upward, and draw in or reject 
water at pleasure. The water at Mobile is usually fresh, 
* Second Visit to the United States, vol. ii., p. 104. 
