FOSSIL CYPRIDES. 305 



The palleal impression extends from one muscular imprint to 

 the other, running parallel with the margin of the base. There 

 is a considerable difference observable in the width of the pos- 

 terior part of the shell in a series of specimens, which appears 

 to be sexual ; for a similar variation occurs in the male and 

 female shells of the recent species from the Ohio. 



The collocation of these large mussels with 

 drifted trees and bones of land reptiles, in clays 

 and sands so manifestly of fluviatile origin, com- 

 pletes the analogy between the rafts imbedded in 

 the delta of the Wealden, and those which float 

 down, and become engulfed in the mud and silt 

 of the Mississippi. 



Fossil Cyprides. — The minute crustaceous ani- 

 mals termed Cyprides, of which the shells or cases 

 of several species are so abundant in the Wealden 

 strata, were brought under the notice of the reader 

 in our examination of the tertiary strata at White- 

 cliff Bay and Headon Hill ; and in the clays and 

 shales of Sandown Bay (p. 136). These animals 

 have the body enclosed in a horny case, consist- 

 ing of two pieces united by a hinge line. They 

 have one compound eye, and four feet, and two 

 straight simple antennae with a tuft of cilia at the 

 extremities. They swim with rapidity, and may 

 be seen actively pursuing the minuter animals on 

 which they prey. Like other crustaceans, they 

 frequently shed their cases, and the surface of the 



