290 



LOWER EOCENE FORMATIONS, ENGLAND. [Ch. XVI. 



distinguishes this member of the series. Among the latter, Melania 

 inquinata (see fig. 263) and Cyrena cuneiformis (see fig. 262) are very- 



rig. 262. 



Fig. 263. 



■ cuneiformis, Min. Con. 

 Natural size. 



Melania inquinata, Des. Nat. size. 

 Syn. Cerithium melanoides, Min Con. 



common, as in beds of corresponding age in France. They clearly indi- 

 cate points where rivers entered the Eocene sea. Usually there is a mix- 

 ture of brackish freshwater, and marine shells, and sometimes, as at 

 Woolwich, proofs of the river and the sea having successively pre- 

 vailed on the same spot. At New Charlton, in the suburbs of Wool- 

 wich, M. de la Condamine discovered in 1849, and pointed out to 

 me, a layer of sand associated with well-rounded flint pebbles in which 

 numerous individuals of the Cyrena tellinella were seen standing end- 

 wise with both their valves united, the posterior extremity of each 

 shell being uppermost, as would happen if the mollushs had died in 

 their natural position. I have described * 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 Gnathodon, which were similarly placed 

 with their shells erect, or in a position which enables the animal to 

 protrude its siphon upwards and draw in or reject water at pleasure. 

 The water at Mobile is usually fresh, but sometimes brackish. At 

 Woolwich a body of river-water must have flowed permanently into 

 the sea where the Cyrence lived, and they may have been killed sud- 

 denly by an influx of pure salt water, which invaded the spot when 

 the river was low, or when a subsidence of land took place. Traced 

 in one direction, or eastward toward Heme Bay, the Woolwich beds 



* Second Visit to the United States, vol. ii. p. 104. 



