268 HISTORICAL PALASONTOLOGY. 
The Lamp-shells (Brachiopods) have now reached a further 
stage of the progressive decline, which they have been under- 
going ever since the close of 
the Palzeozoic period. ‘Though 
individually not rare, especially 
in certain minor subdivisions 
of the series, the number of 
generic types has now _ be- 
come distinctly diminished, the 
principal forms belonging to 
the genera TZerebratula, Tere- 
bratella (fig. 194), Terebratulina, 
: Rhynchonella, and Crania (fig. 
193.—A small fragment of Escharina 195). In the last mentioned 
Oceani, of the natural size; anda portion S 
of the same enlarged. Upper Greensand. of these, the shell 1S attached 
to foreign bodies by the sub- 
stance of one of the valves (the ventral), whilst the other or 
free valve is more or less limpet-shaped. All the above-men- 
O\~/@\ 
fi 
Dot 
Fig. 194.—Terebratella Astieriana. Gault. 
tioned genera are in existence at the present day; and one 
species—namely, Terebratulina striata—appears to be undis- 
tinguishable from one now living—the Zerebratulina caput- 
serpentts. 
Whilst the Lamp-shells are slowly declining, the Bivalves 
(Lamellibranchs) are greatly developed, and are amongst the 
most abundant and characteristic fossils of the Cretaceous 
period. In the great river-deposit of the Wealden, the Bivalves 
are forms proper to fresh water, belonging to the existing 
River-mussels (Unzo), Cyrena and Cycas; but most of the 
Cretaceous Lamellibranchs are marine. Some of the most 
abundant and characteristic of these belong to the great family 
of the Oysters (Ostretde). Amongst these are the genera 
Griyphea and Exogyra, both of which we have seen to occur 
