524 
THE CRETACEOUS ROCKS OF BRITAIN. 
In Dorset and Devon the higher parts of the zone show signs 
of a change of conditions; the chalk becomes full of small 
Foraminifera and Spheres and minute shelly fragments; small 
yellowish grains of glauconite, nearly absent in samples of this 
zone from the midlands and south-eastern counties, make their 
appearance. Sponge-spicules are of frequent occurrence, some 
beds assume the character of a semi-crystalline limestone, and 
the general conditions seem somewhat similar to those of the 
Chalk Lock. 
Of the organisms which occur in the Middle Chalk, Spheres 
and Inoceramus- prisms have already been alluded to, but another 
constant feature deserves notice, viz., the number of minute 
ossicles of peculiar character which have been recognised by Dr. 
Gregory as the ambulacral plates of an Ophiurid. These plates 
are especially numerous in the Terebratulina zone, though they 
occur commonly in the lower zone as well. 
The number of Foraminifera obtained from washings and resi¬ 
dues is not so large as in the Lower Chalk, and those isolated do 
not show much variation when the species of this division are 
compared with those of the Lower Chalk. Judging from the 
results of the washings, Foraminifera are not very abundant, for, 
as we have already stated, the amount of material used was quite 
as large as that from the lower division, and its condition in the 
T.erebratulina zone was favourable to the separation of these 
forms. 
Arenaceous Foraminifera are comparatively rare, but short 
lengths of arenaceous tubes, jDrobably fragments of Bhizammina, 
form a considerable part of many of the residues. The most per¬ 
sistent forms are those of Ammodiscus. Globigerina cretacea 
and Globigerina marginata were both fairly common in the 
washings, but it may be remarked that their numbers are not 
greatly in excess in their proportion to other Foraminifera. 
It is difficult to estimate what proportion they bear to the 
mass of the Chalk; certainly it is not a very large one, and 
individually they are far less abundant than in specimens of Atlantic 
ooze in our possession. Judging from thin sections, they appear 
to be most abundant in the lower part of the Middle Chalk, especi¬ 
ally in such sections as contain many shell-fragments, and where 
the heavier particles seem sorted out, presumably by current action. 
Thin sections also show that the shells of Foraminifera in the 
Terebratulina zone become thin, though the specimens of Globi¬ 
gerina isolated from the lower part of this zone were especially 
fine, larger in fact than from any other horizon in the Chalk. 
Sponge spicules are rare in the Middle Chalk, and are only con¬ 
spicuous in the hard beds at the summit of the division in the 
west of England. 
