Ch. XVIIL] 



FOSSILS OF LOWER GREEXSAXD. 



34:3 



Pebbles of quartzose sandstone, jasper, and flinty slate, together 

 with grains of chlorite and mica, speak plainly of the nature of the 



Fig. 830. 



Perna MulUU. Desh. and Leym. 

 a. Exterior. b. Part of hinge of upper valve. 



preexisting rocks, from the wearing, down of which the Greensand 

 beds were derived. The land, consisting of such rocks, was doubt- 

 less submerged before the origin of the white chalk, a deposit which 

 originated in a more open sea, and in clearer waters. 



The fossils of the Lower Cretaceous are for the most part specific- 

 ally distinct from those of the Upper Cretaceous strata. 



Among the former we often meet with the genus Scaphites or 

 Ancyloceras (fig. 331), which has been aptly described as an ammo- 



Fig. 331 



j 

 Ancyloceras gigas, D'Orb. 



Nautilus plicatw, Sow., in 

 Fitton's Monog. 



nite more or less uncoiled ; also a farrowed Nautilus, JV. plicatus 

 (fig. 332), Trigonia caudata (fig. 334), likewise found in the Black- 

 down beds (sec above, p. 332), and Gervillia, a bivalve genus allied to 



