406 



THE WONDERS OF GEOLOGY. Lect. IV. 



trine and marine forms cannot be distinguished in a fossil 

 state : the nature of the originals can only be inferred by the 

 organic remains with which they are collocated. Like 

 other crustaceans they frequently shed their cases, and the 

 mud spread over the bottoms of still lakes is often covered 

 with their exuviae.. The largest living Cypris does not 

 exceed one sixth of an inch in length. In a fossil state their 

 cases appear like white elliptical reniform scales, on the 

 surface of the recently separated laminae of clay, shale, and 

 limestone. Upon exposure to the air the cases decompose, 

 and leave the surface of the shale or stone covered with the 

 casts of the shells, which appear as minute polished 

 tubercles ; some layers of the indurated ironstone have a 

 granulated surface from these remains.* In the Sussex 

 and Purbeck marbles, as previously mentioned, the cyprides 

 are most abundant. There are several fossil species, one of 

 which is represented very highly magnified in Lign. 98, 

 figs. 1, 3. 



As the recent species inhabit either still lakes or gently 

 flowing streams, and not the turbulent waters of estuaries, 

 it follows that sediments largely charged with these exuviae, 

 associated with freshwater shells, afford strong presump- 

 tive evidence of a fluviatile origin. 



27. Insects op the Wealden. — The durable nature 

 of the wing-cases (elytra), legs, antennae, &c. of the co- 

 leoptera, and other insects, rendered it probable that some 

 vestiges of this class of articulated animals would be pre- 

 served in the muddy sediments of the Wealden. But 

 I sought in vain for any vestiges of this kind, in the localities 

 of the south-east of England that were accessible to my 

 observations. In the remote outliers of the Purbeck beds 

 in the vale of Wardour in Wilts, and in several places in 



* In the sandstone quarries at Langton Green, near Tunbridge 

 Wells, slabs of this kind occur, studded over with casts of cyprides, 

 and minute unioncs. 



