414 
PLIOCENE FOSSILS. 
The Coralline or White Crag, tlie most ancient of the Ea.st Anglian 
deposits, is composed of shelly sand and marls, the term coralline 
being due to the inimen.se (piantity of coral-like polyzoa it contains, 
mail}'' of which still retain the iiosition they occupied when living. 
The Red Crag is formed by local beds of dark-red or brown ferru- 
ginous sand, tlieir colour being derived from iron oxides; they re.st 
nnconformably against and upon the White Crag. One of the most 
interesting shells of this formation is Fasitf: contrarius, a form closely 
allied to which still exists near Vigo Bay. 
The Norwich or Mammalian Crag consist also of shelly sand and 
gravel, attaining in places a thickness of from loO feet to 180 feet. 
The name is derived from one of the localities where the beds are 
well exi)osed, and the descriptive name is in allusion to the presence 
of bones of the iMastodon and other mammalia which are found in 
places at the base of the deposit. 
The Pliocene fossils of species now extinct in the British Isles are : 
IIKLICID.E. 
lldix fr II Hr 1(1)1 ^Nliiller, 
/eiix Fer. . 
lartrrf .Miiller, 
iiiaininfii Miiller, 
riiliii/iiinxa A. Seluiiiilt, 
ri/xfi S. V. Wood, 
siittoiiciixix S. \'. Wood, 
I’UI’ID.E. 
Cldiisiliti plioceud S. y. Wood, 
P.VLUDE.STRINID.E. 
PaliKlcstrinii ohtiixii v. rccn’l K.&W. , 
jiciidiiln S. V. Wood, 
h'reliclldtd (Nyst), 
vivii’.Mtin.n. 
Vii’i/idi'd i/ldcidlis (S. V. Wood), 
iiu'dia (S. Woodward), 
COUBICULID.E. 
t'urbicida flitminalis (Miiller). 
'I’lie PostTertiary strata maybe considered as inaugurating afourth 
or (|naternary epoch, and as linking the life of the Pliocene iieriod 
with tliat of tlie ju’esent day; its beds are composed of various super- 
ficial deposits in which all or nearly all the mollusca are recent species. 
Tlie geological history of our British species is in its origin still 
wrapped in obscurity, as tlie pleistocene remains though showing certain 
.species now extinct in this country, exhibit also many of our present 
species, apparently as far as the shell enables us to judge, quite as 
sharply differentiated as at the present day, so that we must look 
far into the jiast before we can hope to meet with relics of the pro- 
genitors of the present molluscan fauna of our country, as we have 
no distinct traces left behind even of the various stages, as shown by 
the embryonic whorls, through which the iiresent specialization and 
perfection of their shell structure has been acquired. 
These, post tertiary beds may be separated into two groups, which 
are distinguished as Pleistocene and Holocene. 
