— 15 = 
the amorphous silica is, for the most part, changed into crypto- 
crystalline chalcedony, which has also filled up and obliterated 
the interior canals. They thus furnish corroborative evidence 
of the instability of silica in its amorphous condition, and show 
that even where sponge spicules have been preserved tolerably 
complete in outer form yet their interior structure has been 
considerably modified. A similar change has also been 
noticed in the mineral condition of sponge spicules from other 
formations and localities. Thus Mr. Carter (An. and Mag. 
Nat. Hist. S. 4, Vol. 7, p. 112 et seq.) describes the spicules 
from the upper Greensand of Haldon near Exeter as being 
changed into chalcedony and having their surfaces eroded 
and the interior canals partly or wholly destroyed. The 
condition of the spicules from the flints of the North of Ire- 
land, discovered by Mr. Wright, is not stated (List. of the Cre- 
taceous Microzoa of the North of Ireland), but as the calca- 
reous remains in these flints were infiltrated with silica, like 
those from Horstead, it may be inferred that the sponge spic- 
ules had been altered in a similar manner. I find too, that 
a similar alteration in the mineral condition of the silica has 
taken place in sponge spicules forming thin beds in the Lower 
Green Sand (Neocomian) in Surrey, which I have . lately dis- 
covered. This change, however, in the condition of spicules, 
occurring, as in the instances mentioned, detached and scattered 
in the interior of flints or in beds of Green sand, does not 
appear to have taken place invariably, and a noted exception 
is seen in spicules found in connection with Coeloptychium in 
the upper Chalk Strata of Coesfeld in Westphalia. These 
have been described and figured by Professor Zittel in his 
work on Coeloptychium (Abh. d. k. Ak. d. Wiss. XII Bd., 
lil. Abth.) and present a great contrast to the spicules from 
Horstead. The Coesfeld spicules have, by reflected light, 
a smooth, white porcellanic aspect, and when mounted in 
Canada balsam, their outlines under the microscope -are_ still 
seen to be smooth, and the interior canals are equally as per- 
