556 
THE CRETACEOUS ROCKS OF BRITAIN. 
water genus Cystechinus, and as there is an absence of specially 
littoral genera, the Echinoderm fauna of the Middle Chalk has 
the aspect of a fairly deep water assemblage. 
Spongida. —Siliceous sponges are common in the Middle 
Chalk, especially in that of the southern counties, and they 
belong chiefly to the Hexactinellids, the commonest genera being 
Graticularia, Guettardia, Plocoscyphia, Cephalites and Ventri¬ 
culites. Dr. Rowe remarks that sponges of these and other 
genera are more abundant in the Terebratulina zone at Dover 
than in any other part of the Chalk, or in any other exposure of 
this zone known to him.* They also reach a large size in this 
zone, one Guettardia which he measured being 14 inches across, 
and a mass of Craticularia Fittoni was a foot square. 
Hexactinellids are essentially deep-water sponges, and in 
modern seas they appear to be most abundant in depths of 
300 and 1,000 fathoms. 
Foraminifera. —The foraminiferal fauna of the Middle Chalk 
has not yet been studied in much detail, but it needs only a brief 
examination of a few samples to discover that the tests of arenaceous 
Foraminifera are much less common than in the Lower Chalk. 
As stated on p. 287, in the Chalk Marl fine specimens of Textularia, 
Tritaxia, Gaudryina, Bulimina, Bolivina, and other arenaceous 
genera are abundant; but in the higher beds they become much 
rarer. This was first pointed out by Dr. W. F. Hume, and with 
regard to the Middle Chalk he wrote, “ in the lower zones of the 
Middle Chalk all the more coarsely-tested forms have entirely 
disappeared, and only the minutest Bulimines, a most delicate 
little Textularia, and the ammonitoid shells of Ammodiscus incer- 
tus, survive to represent the arenaceous species.” + 
Mr. Chapman’s determinations of the species in the numerous 
washings and residues of Middle Chalk prepared by Mr. Hill, 
furnish a longer list of species, but still it is a short one compared 
with that of the Lower Chalk species, and it supports Dr. Hume’s 
main contention for the arenaceous forms are chiefly species of 
Ammodiscus with small Bulimines and Textularians. 
In the zone of Rhynchonella Cuvieri eighteen species have been 
identified belonging to genera which live on the sea-floor. Of 
these two are extinct and of the remainder no less than twelve 
are species which now range to depths of more than 1,000 fathoms. 
Only four species have been found which now have a limited 
range ; these are Rhizammina indivisa, which has not occurred 
*See Proc. Geol. Assoc. Vol. xvi., p. 317 (1900). 
I t Natural Science, Vol. vii., p. 273. .. 
