296 Mr. W. J. Sollas on the Action of Caustic 



within outwards and the outermost lamellse usually endure 

 longest (fig. 19, a). 



The smaller spicules of this Trachya have been entirely dis- 

 solved by the solution, which has merely analyzed the larger 

 ones. 



Halichondria incrustans and H. panicea. 



The flesh-spicules (anchorates and tricurvates) of the former 

 species soon passed into solution ; and in the skeleton-spicules 

 of both, the axial canals very quickly appeared and rapidly 

 enlarged. A concentric lamellar structure was exposed as in 

 the cases we have previously described ; but in these small spi- 

 cules the number of component laminae appears to be very 

 small, not above two or three at the most. One has to be 

 careful in attending to the solution of these spicules, not to let 

 them boil too long ; otherwise they may all dissolve away : a 

 boiling sufficient to develop the spicules of the Trachya of the 

 preceding paragraph completely dissolved all or nearly all the 

 spicules of a specimen of H. incrustans. From this one might 

 conclude that a search for fossil spicules of so small a size as 

 these would not, unless under very favourable circumstances, 

 be likely to be attended with success. 



This concludes the account of the observations on the action 

 of caustic potash which I have thought it worth while to 

 record ; and it only remains to add a i^w words by way of 

 application and explanation. And first as to the curious fact 

 that solution appears to proceed much more rapidly in the 

 interior than on the outside of spicules or spicular fibre, so 

 that very frequently we find the exterior of a spicule per- 

 sisting as a thin shell of apparently the same diameter as it 

 had originally, while interiorly every thing has been dissolved 

 away. It may be said that the difficulty here is more appa- 

 rent than real, since the internal solution takes place under 

 circumstances very favourable for observation, while that of 

 the exterior is less easily made manifest. When it reveals 

 itself in the edges of eroded laminaa (PI. IX. fig.l9, a &h) \i is 

 obvious enough ; but in most instances its effects cannot so 

 readily be estimated. There is no doubt some truth in this ; 

 but in many cases the difference in the rate of the internal and 

 external solution is too great to be accounted for in such a 

 manner. Capillarity will not help us much ; for it could only 

 lead to the introduction of the potash into the fine axial canal 

 and between the successive concentric lamellee of the spicule, 

 and assist solution so far only as it might do so by bringing 

 the solvent agent into every accessible crevice of the dis- 



