§ 26. CRUSTACEANS OF THE WEALDEN. 405 



and those brought down, and engulfed in mud and sand, 

 by the Ohio and Mississippi, and other large rivers. 



Lign. 98. — Crustaceans and shell, prom the Wealden. 



Figs. 1 and 3. Cypris granulosa; highly magnified. 

 2. Mytilus Lyellii; one-third natural size. 



A few small freshwater mussels occur in the clay at 

 Pounceford, associated with cyclades ; a species is figured 

 in Lign. 98, fig. 2. 



26. Crustaceans of the Wealden.* — The crusta- 

 ceans of the Wealden, though occurring in inconceivable 

 numbers, belong to but two kinds ; viz. Archoeoniscus and 

 Cypris. The first are referable to the isopodous order, of 

 which our common Oniscus, or wood-louse, is an example ; 

 but the fossil Isopoda were aquatic. They were discovered 

 by Mr. Brodie, with remains of insects, in grey marly 

 Purbeck limestone, near Dinton.f 



The Cyprides are very small crustaceans, having 

 the body inclosed in a horny bivalve shell or case, 

 united by a hinge, and admitting the antennae and 

 feet to protrude and be at liberty, when the case is 

 open. They have only one eye, situated in the middle 

 of the head ; the species which abound in our pools of 

 fresh-water have two pairs of pats or feet ; those of 

 brackish and salt-water ( Cytherina) have three pairs, and 

 inferior antennas ; but the shells are so alike, that the lacus- 



* Medals of Creation, vol. ii. p. 544. 



f Ibid. p. 542; and History of Insects, p. 10. 



