PART II. CHAPTER XVI. 



201 



Wealden Group Fossils. 



Fig. 181. 



marine 



freshwater 



Weald Clay, ) 

 Hastings sand, > Wealden. 

 Purbeck beds, ; 



Oolite. 



Position of the Wealden between two marine formations. 



Fossils of the Wealden. The shells of this formation are 

 almost exclusively of fluviatile or lacustrine genera, such as Me- 

 lanopsis, Paludina, Neritina, Cyclas, Unio, and others. The indi- 

 viduals are sometimes in such profusion, that the surface of each 

 thin layer of marl or clay is covered with the valves of Cyclas, and 

 whole beds of limestone are almost entirely composed of Palu- 

 dinse. Intermixed with these freshwater shells, there are a few 

 which seem to mark the occasional presence of salt water, as for 

 example, a species of Bulla, together with an Oyster, and the 

 Exogyra, a genus of unimuscular bivalves allied to the oyster (see 

 Fig. 182.). The conclusion to be drawn from the presence of a 

 Corbula (see Fig. 183.) and Mytilus is more doubtful; for al- 



Fig. 182. 



Fig. 183. 



Exogyra bulla. Fitton. 



Corbula alata. Fitton. Magnified. 



though these genera are for the most part marine, still there is a 

 Mytilus living in the Danube, and one species of Corbula in- 

 habits the river La Plata, in South America, as well as the ad- 

 joining sea, while another is common to the Caspian, and the 

 rivers Don and Wolga. But admitting all these to have been 

 marine, they by no means outweigh the evidence, both of a posi- 

 tive and negative kind, derived from shells in favour of the fresh- 

 water origin of the Wealden. In no part of this deposit do we 

 meet with ammonites, belemnites, terebratulse, corals, sea-urchins, 

 or other testacea and zoophytes so characteristic of the chalk 

 above, or the oolite below the Wealden. 



