— 41 — 



smaller o\al canals arranged at the same distance from 

 the edge of the disc. 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 0,27 mm., and the shorter 0,18 mm. 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 

 greater 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- 

 dcrmia and other allied Lithistid .sponges, named them 



