MIDDLE CHALK—CONDITIONS OF DEPOSIT. 557 
below 540 fathoms, G-audryina filiformis , which ranges to 620 
fathoms ; Textularia trochus and T. turris , which have not been 
obtained below 440 fathoms. All these, however, are rare species 
in the zone of Rhynch. Guvieri. 
Among the specimens obtained by Mr. Hill from the Tere- 
bratulina zone, Mr. Chapman found twenty-eight which could 
be specifically named. Of these six are not known as recent 
species, thirteen have a great range in depth, three are species 
of Globigerince, and there remain only six to connect the fauna 
with that of shallower water. These are : — 
Rhizammina indivisa. Tritaxia tricarinata. 
Textularia trochus. Gaudryina filiformis. 
„ turris. ! ,, rugosa. 
In this zone Rotalia soldani (var. nitida ) and a species of Pulvinu- 
lina occur, and these are deep water forms. 
From the above analysis it would certainly appeal* that the 
foraminiferal fauna of the Middle Chalk, when compared with 
that of the Lower and even with the higher part of the Lower 
Chalk, suggests water of a greater depth. This inference is sup¬ 
ported by the character of the Ecliinoderm fauna, by the scarcity of 
Bryozoa and by the abundance of Brachiopoda and Siliceous 
Sponges. Hence, if the depth at which the Chalk Marl was 
formed approximated to 400 fathoms, and that of the higher 
beds of the Lower Chalk to 500 fathoms, it is very probable that 
during part of the Middle Chalk time the depth exceeded 500 
fathoms ; but as w^e shall see hereafter there seems to have been a 
recovery by upheaval during the formation of the Chalk rock 
(zone of Holaster planus), consequently the time of greatest depth 
was probably that when the lower part of the Terebramlina zone 
was being accumulated. 
