MERKILL: FOSSIL SPONGE SPICULES. 9 



experiment 1 that the globo-stellates dissolved from within outwards, and 

 the central cavity enlarged from within outwards, until the outer cover- 

 ing became a mere lilm. The spicule shown in Figure 22 is a good 

 illustration of this fact in nature. The middle portion, not figured, is 

 detached from the spines, and 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." a 



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 in all the 

 slides examined. These small flesh spicules are not common in the de- 

 posits of England. 3 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 elongated ami 

 tapering at one end. Similar replacements have been described by Dr. 

 Hinde, 4 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 appe.tr- 



1 Annals and Magazine of Natural History, Ser. 4, 1877, Vol. XX. p. 229. 



2 Ibid., Ser. 5, 1880, Vol. VT. p. 445. 



3 Ibid., Ser. 4, 1871, Vol. VII. p. 122; also Ser. 5, 1880, Vol. VI. pp. 441, 442- 

 * Catalogue of Fossil Sponges of the British Museum, p. G. 



