MERRILL: FOSSIL SPONGE SPIOULES. 9 
experiment?! that the globo-stellates dissolved from within outwards, and 
the central cavity enlarged from within outwards, until the outer cover- 
ing became a mere film. The spicule shown in Figure 22 is a good 
illustration of this fact in nature. The middle portion, not figured, is 
detached from tho spines, aud has almost entirely disappeared, leaving 
the spines surrounding the centre in the position formerly sustained. 
This is plainly a case of solution in which the spines have survived 
the globate centre. It is a peculiar fact, also, that although much of the 
‘spicule has been dissolved, yet there has been no replacement by crystal- 
line silica, but the surrounding amorphous silica shows no trace of the 
original outlines of the dissolved portion. ‘This may also explain the fact 
that the spicules (Figs. 1 and 2) merge so gradually into the surrounding 
amorphous silica that it is impossible to tell exactly where the spicule 
ends. It thus seems to occur frequently that the spicule is dissolved, 
and redeposited as amorphous silica without definite form. 
Professor Sollas says that ‘ spicules of sponges are colloidal because 
of spiculin, and being dissolved they lose their spiculin and are rede- 
posited as crystalline silica, and may be amorphous or chalcedonic." ® 
The solution and redeposition of masses of spicules is a simple process, 
but the principle, if there be one, that enables some to be replaced while 
thousands are entirely dissolved, is more difficult to determine. The 
flesh spicules seem to have resisted the processes of solution better than 
the others, and are more numerous, as well as more perfect im all the 
slides examined. These small flesh spicules are not common in the de- 
posits of England." Their abundance and perfection in the flints of 
Texas, therefore, indicate a difference in the surrounding conditions that 
is not altogether apparent. Another form of fossil spicule, more rare 
as a whole, but common in some of the nodules, is replacement by per- 
oxide of iron. This is not found"continuous throughout a whole spicule, 
but is usually an irregular, disconnected chain of dark bead-like masses. 
These masses are generally globular in shape, but often clongated and 
apering at one end, Similar replacements have been described by Dr. 
Hinde,* and have also been referred to by others. These spicules occur 
in various degrees of completeness, the most perfect found being Figure 
34, in which there seems to be a peculiar mixture of amorphous silica 
and peroxide of iron alternating, thus giving the spicule a spotted appoar- 
1 Annals and Magazine of Natural History, Ser. 4, 1877, Vol. XX. p. 229. 
2 Ibid., Ser. 5, 1880, Vol. VT. p. 445. 
8 Ibid., Ser. 4, 1871, Vol. VII. p. 122; also Ser. 5, 1880, Vol. VI. pp. 441, 442. 
4 Catalogue of Fossil Sponges of the British Museum, p. 6. 
