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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 spectes, 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, ta 
facilitate reference in the future. 
Of the works which have already appeared on detached 
fossil sponge spicules to which I shall frequently have occasion 
to refer, the. first is that of Mr.-H. ]i-Catter sR aR. Siir@a 
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 
