296 
ELEMENTS OF GEOLOGY. 
Fig. 25T. 
a. Natural size. 6. Portion magnified. 
(Fig. 238, p. 292), are dispersed indifferently through the 
soft chalk and hard flint, and some of the flinty nodules owe 
their irregular forms to inclosed 
sponges, such as Fig. 25 9, a, where 
the hollows in the exterior are 
caused by the branches of a 
sponge (Fig. 259, b), seen on 
breaking open the flint. 
The remains of fishes of the 
Upper Cretaceous formations 
consist chiefly of teeth belong¬ 
ing to the shark family. Some 
of the genera are common to the 
Tertiary formations, and some 
are distinct. To the latter be¬ 
longs the genus Ptychodus (Fig. 260), which is allied to the 
living Port Jackson shark, Cestracion PMllippi^ the anterior 
Escharina oceani. 
a Natural size, b Part of the same 
magnified. White chalk. 
5 Fig. 259o a 
A branching sponge in a Hint, from the white chalk. From the collection 
of Mr. Bowerbank. 
teeth of which (see Fig. 261, a) are sharp and cutting, while 
the posterior or palatal teeth (b) are flat (Fig. 260). But we 
meet with no bones of land-animals, nor any terrestrial or 
fluviatile shells, nor any plants, except sea-weeds, and here 
and there a piece of drift-wood. All the appearances concur 
