smaller oval canals arranged at the same distance from 
the edge of the dise. There is no trace of any canals radiating 
from the centre. These discs are tolerably uniform in size 
and have an average diameter of 0,315 mm. 
The other form of disc (fig. 24) is likewise extremely 
thin; elliptical in form, and having the outer edge deeply 
notched or scalloped at regular intervals. In most of the 
specimens the canals are obliterated, but one fragmentary disc 
(fig. 26) shows about 18 straight canals radiating from the 
centre towards the circumference. The long diameter of these 
specimens measures 0,27mm., and the shorter 0,18mm. Both 
forms are rare. 
Professor Zittel has described and figured similar discs 
from the Upper Chalk of Westphalia, (Ueber Coelop. p. 47. 
Taf. V. figs. 32—-35) and he is disposed to regard the discs 
with the scalloped edges as the incomplete forms of those 
with the even border. The Westphalian specimens lend 
ereater probability to this supposition than the Horstead 
examples, for some of the even bordered discs from West- 
phalia have, in addition to the flat and oval shaped canals 
near the border of the disc, a variable number of canals 
radiating from the centre to the circumference similar to those 
in the scalloped discs. In the Horstead specimens, on the 
other hand, the canals radiating from the centre only appear 
in discs of this latter form, and as these are all elliptical in 
outline as well, in contrast to the circular’ even bordered 
discs, I think that these two forms of discs may be considered 
as separate from, though closely allied to each other. 
Similar discs with numerous radiating canals have not 
yet been discovered in existing sponges, so that there is no 
direct clue to the affinities of these small bodies. Mr. Carter 
discovered two of the even bordered discs in the Haldon 
Green Sand and regarding them as allied to the circular and 
branched discs which form the surface spicules of Disco- 
dermia and other allied Lithistid sponges, named them 
