THE CRETACEOUS PERIOD. 289 
poda are the most valuable, as having a more restricted 
range in time than the Gasteropoda; and these, again, are 
more characteristic of the particular stratigraphical subdh 
visions than are the Lamellibranchiate Bivalves, while these 
last, again, are more serviceable in classification than the 
Brachiopoda, a still lower class of shell-fish, which are the 
most enduring of all. 
When told that the new dredgings prove that “we are 
still living in the Chalk Period,” we naturally ask whether 
some cuttle-fish has been found with a Belemnite forming 
part of its internal frame-work; or have Ammonites, Bacu- 
lites, Hamites, Turrilites, with four or five other Cephalopo- 
dous genera characteristic of the chalk and unknown as ter¬ 
tiary, been met with in the abysses of the ocean ? Or, in the 
absence of these long-extinct forms, has a single spiral uni¬ 
valve, or species of Cretaceous Gasteroj)od, been found liv¬ 
ing ? Or, to descend still lower in the scale, has some char¬ 
acteristic Cretaceous genus of Lamellibranchiate Bivalve, 
such as the Inoceramus, or Hippurite, foreign to the Tertiary 
seas, been proved to have survived down to our time ? Or, 
of the numerous genera of lamellibranchiates common to the 
Cretaceous and Recent seas, has one species been found liv¬ 
ing? The answer to all these questions is — not one has 
been found. Even of the humblest shell-fish, the Brachio- 
pods, no new species common to the cretaceous and recent 
seas has yet been met with. It has been very generally ad¬ 
mitted by conchologists that out of a hundred species of this 
tribe occurring fossil in the Upper Chalk—one, and one only, 
Terebratidina striata^ is still living, being thought to be iden¬ 
tical with Terebratula capiit-serpentis. Although this iden¬ 
tity is still questioned by some naturalists of authority, it 
would certainly not surprise us if another lamp-shell of equal 
antiquity should be met with in the deep sea. 
Had it been declared that we are living in the Eocene 
epoch, the idea would not be so extravagant, for the great 
reptiles of the Upper Chalk, the Mososaurus, Pliosaurus, and 
Pterodactyle, and many others, as well as so many genera 
of chambered univalves, had already disappeared from the 
earth, and the marine fauna had made a greater a]3proach to 
our own by nearly the entire difference which separates it 
from the fauna of the Cretaceous seas. The Eocene num- 
mulitic limestone of Egypt is. a rock mainly composed, like 
the more ancient white chalk, of globigerine mud; and if 
the reader will refer to what we have said of the extent to 
which the nummulitic marine strata, formed originally at 
the bottom of the sea, now enter into the frame-work of 
13 
