Lower Greensand. 



217 



side of the great Chalk escarpment, between Devizes 

 and the Wash, the Lower Greensand is often ferruginous, 

 and has been worked for iron ore both in ancient and 

 modern times. Fossil wood is of frequent occurrence, 

 perhaps of Coniferous trees, and all the evidence tends 

 to show that, in the English area, the strata were 

 deposited in comparatively shallow seas not far from 

 shore. 



The general characters of the fossils of the series are 

 as follows : Echinoderms of the genera Salenia, Car- 

 diaster, Diadema, Discoidea, Echinobrissus, together 

 with Pentacrinites, are found in it. Terebratulce and 

 Rhynchonellce are of frequent occurrence, with a few 

 other Brachiopoda. Among the Lamellibranchiate 

 molluscs are numerous Limas, Gervillias, Perna, 

 Oysters, Pectens, and Pinnas, together with shells of 

 the genera Cardium, Venus, Trigonia, Myacites, 

 and Nucula. Gasteropoda are not generally numer- 

 ous. Cephalopoda of remarkable forms are character- 

 istic ; for, in addition to several species of Ammonites, 

 Nautili, and Belemnites, there are Crioceras, and 

 Ancyloceras, like Ammonites half unrolled, Crioceras 

 Bowerbankii, Ancyloceras gigas, A. grande, and A. 

 Hillsii. Fishes are scarce, and only three reptiles 

 have hitherto been described, one Chelonian. Protemys 

 serrata, a Plesiosaurus, and a crocodilian saurian 

 Polyptychodon continuus, said also to occur in the 

 Lower Chalk. 



Out of about 300 Lower Greensand species, 18 or 

 20 per cent, pass into the Upper Cretaceous series. 

 Partly for palseontological considerations, and also 

 because the Gault seems sometimes to lie, as it were, 

 unconformably on the eroded surface of the sand, the 

 dissimilarity in the grouping of fossils is so great, that 



