18 



spicule being common to several different genera of sponges 

 so that it is quite impracticable to determine to which or 

 how many of these genera the form may have belonged. 

 In other sponges, there are six or seven different forms of 

 spicule in the same species, of which one or two of the forms 

 may be characteristic of the species, whilst the other spicules 

 may be common to other species as well, so that, although 

 all the different forms of spicule may be present in the ma- 

 terial, there is no certainty that they have been derived from 

 the same genus or species of sponge. The peculiar form 

 and mode of attachment of the spicules of the Lithistid and 

 Hexactinellid Sponges permits of a better identification of their 

 spicules than in those of the Tetractinellid sponges, and in 

 several instances the correspondence in form and size of the 

 spicules is so close to that of sponges already determined, 

 that no doubt can arise of their belonging to the same species. 

 Under these circumstances I have arranged these spicules under 

 the different genera with which they seemed to have the 

 closest relationship, and in only a few exceptional cases in 

 which the peculiar form or dimensions of the spicule rendered 

 it highly probable that it belonged to some hitherto unre- 

 cognized sponge , have I ventured to give a name to it , to 

 facilitate reference in the future. 



Of the works which hav^ already appeared on detached 

 fossil sponge spicules to which I shall frequently have occasion 

 to refer, the first is that of Mr. H. J. Carter, F. R. S. On 

 Fossil Sponge Spicules of the Greensand compared with those 

 of existing Species. (Annals and Mag. Nat. Hist. 1871, S. 4 

 Vol. 7, p. 112, Plates VII X.) In this article, Mr. Carter 

 gives an excellent description and figures of 76 different forms 

 of detached spicules which were discovered in strata of the 

 Greensand Formation (Cenomanien) at Haldon near Exeter, 

 and shows their close relationship to existing sponges. 

 Some of these Greensand spicules are identical in form with 

 those from the Horstead flint and all of them are closely 



