44 



X 



Such differences of size and form indicate that these spi- 

 cules may have belonged to several species of sponge and 

 this is rendered further probable by the great numbers which 

 are present in this Horstead flint. 



I have some difficulty in determining the relationship of 

 this group of spicules with existing forms. The compound 

 trifid spicule with the head expanded horizontally frequently 

 forms the zone spicule of Geodia and Stelletta, but accom- 

 panied by an elongated shaft; a similar form of spicule is 

 present in the dermal portion of many species of Lithistids, 

 but these are so diminutive in comparison with those figured 

 as to negative the probability of their being surface spicules of 

 these sponges ; I also compared them with the truncated spicules of 

 Pachastrella, but these have the arms mostly simple, or where bifur- 

 cated, the division is generally restricted to the ends of the arms ; at 

 last I found the description of a sponge with a similar spicule, 

 placed by Mr. Carter under the name of Tisiphonia nana(An, Mag. 

 Nat. Hist. S. 5, Vol. 6, p. 492, PI. VII, fig. 43 a, b). This spicule 

 has a truncated shaft and an expanded head closely resem- 

 bling some of the Horstead forms. Further, O. Schmidt has 

 figured the head of a spicule similar to that of fig. 20 in a 

 sponge from the Adriatic under the name of Stelletta Hilleri 

 (Supp. d. Spong. Adriat. Meeres, p. 32, Taf. Ill, fig. 8) and 

 as according to Mr. Carter the genus Stelletta is equivalent 

 to Tisiphonia, there is a probability that these spicules with 

 truncated shafts may be the zone spicules ot sponges allied 

 to Stelletta, notwithstanding the great differences which they 

 present, in the comparative reduction of the shaft and the 

 extended development ot the head, from the typical zone 

 spicules of Stelletta itself. 



Similar spicules but as a rule very much smaller in di- 

 mensions, have been found by Mr. Carter in the Haldon 

 Green Sand (An. Mag. Nat. Hist, S. 4, Vol. 7, PL IX, 

 figs. 30 36) ; they are also present in the Chalk of the 

 North of Ireland (Wright op. cit. PI. II, figs. 17 a, b) and 



