42 



% 



Dactylocaly cites callodiscus (An. Mag. N. H. S. 4 Vol. 7 p. 

 123 PL IX. fig. 40). It seems to me however, that the 

 affinities of these discs is rather with the siliceous globules 

 and stellates of Geodia and other genera of Tetractinellid 

 sponges than with the Lithistidae. In the surface spicules of 

 both recent and fossil Lithistids, in which canals are present, 

 they are restricted to three, which afterwards bifurcate in some 

 instances. In no known instance of ascertained Lithistid sur- 

 face spicule is there the number and disposition of the canals 

 as in these isolated discs. Considerable light is thrown upon 

 the affinities of these discs from the description which 

 O. Schmidt has given of the different stages of growth of 

 certain thin oval discs present in the existing sponge Stelletta 

 Euastrum (Spong. d. K. von Algier, p. 20, Taf. IV, fig. 4). 

 In the earliest stage of the development of these discs there 

 is a granular centre from which a single layer of extremely 

 delicate needle-like rays or spicules radiate. More developed 

 specimens show these fine ray-like spicules united near their 

 bases and deeply notched at the periphery. In the complete 

 disc one surface is smooth, flat, or slightly concave, whilst 

 the other is convex and thickly covered with small warts. 



The structure of these discs from the Upper Chalk may 

 be understood, if we suppose that in a similar manner to 

 those of Stelletta Euastrum, they have been developed by 

 the amalgamation of a single layer of very fine spicules ra- 

 diating from a centre and that the shorter and longer canals 

 which are shown in the discs are those of the spicular rays 

 of which they are composed. Although O. Schmidt does not 

 mention the presence of canals in the spicular rays of the 

 discs of Stelletta Euastrum, he yet states that he has repeat- 

 edly noticed a central canal in the similar and equally mi- 

 nute spicular rays of the siliceous globules of Geodia canali- 

 culata (Spong. d. K. von Algier, p. 21). There appears there- 

 fore a probability that these small bodies are the surface discs 

 of a species of Stelletta. 



