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the amorphous silica is, for the most part, changed into crypto- 

 crystalHne chalcedony , wliich has also filled up and obliterated 

 the interior canals. They thus furnish corroborative evidence 

 of the instabilit>- 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. 1 1 2 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., 

 III. 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- 



