~ 
Parr IV. Srov. iv.§2.] | PLIOCENE. Po, er 
§ 2. Local Development. 
Britain.—In the Pliocene period, after a long period of exposure as 
a land surface during which a continuous and ultimately stupendous 
subaerial denudation was in progress, Britain underwent a gentle but 
local subsidence. We have no evidence of the extent of this depression. 
All that can be affirmed is that the south-eastern counties of England 
began to subside, and on their submerged surface some sandbanks and 
shelly deposits were laid down, very much as similar accumulations now 
take place on the bottom of the North Sea. These formations, termed 
“Crag,” are subdivided, according to their proportion of living species 
of shells, into the following groups: 
Forest Bed group. : L . 10 to 70 ft. 
: 4 Chillesford Clay . : : J eee Soe 
poe ee beds {Chillesford Sand with shells . D ate 2 
Norwich Crag. ; : : é : : : ; cof th, eh Ors. 
Red Crag : : ; : 25 55 
White Crag. ; : A GOS 
The W hite Crag (Suffolk, Coralline, or Bryozoan Crag), consisting 
of shelly sands and marls, is exposed in many places in the county 
of Suffolk. It contains 316 species of shells, of 
which 84 per cent. are still living. Among its 
characteristic forms are Terebratula grandis, Lin- 
gula Dumortieri, Pecten opercularis, Pholadomya 
histerna, Astarte Omalii (Vig. 421), Pyrula reti- 
culata, Voluta Lamberti (Fig. 422), Fascicularia 
aurantium (Fig. 420). The name “ coralline” was 
given to the formation from the immense number 
of coral-like polyzoa which it contains, no fewer 
than 140 species having been described. 
The Red Crag is also a thin and local Fic. 420.—Putocenz Pory- 

formation, consisting of a dark-red or brown ZOON. 
ferruginous shelly sand. Of its molluscs, 92 per  Fascicularia aurantium 
cent. are believed to be still living species, and, (M. Edw.) (). 
out of 25 species of corals, 14 are still natives of 
British seas. Some of the typical shells of this subdivision are Trophon 
antiquum (Fusus contrarius, Fig. 422), Voluta Lamberti, Nassa reticosa, 
Purpura lapillus, P. tetragona, Pecten opercularis, Pectunculus glycimeris, 
Mactra arcuata, M. ovalis, Tellina obliqua, Cardium edule, Mytilus edulis, 
and Cyprina rustica. Numerous mammalian remains have been obtained 
from these sands, including bones of Mastodon arvernensis and M. tapiroides, 
Elephas meridionalis, Rhinoceros Schleiermacheri, Tapirus priscus, Sus anti- 
quus, Equus plicidens, Hipparion, Hyzna antiqua, Felis pardoides, Cervus 
anoceros, Halitherium, &c. There is reason to think that some of these 
remains may have been derived from the destruction of Miocene deposits. 
The Norwich, Fluvio- marine, or Mammaliferous Crag 
consists of a few feet of shelly sand and gravel, containng, so far as 
known, 139 species of shells, of which 93 per cent. are still hving. About 
20 of the species are land or fresh-water shells. The name of mammali- 
ferous was given from the large number of bones, chiefly of extinct species 
of elephant, recovered from this deposit. These fossils comprise Mastodon 
arvernensis, Elephas meridionalis, EH. antiquus, Hippopotamus major, Rhino- 
