11-J- Mr. H. J. Carter on Fossil Sponge-sj)icules of 



True enough as this is, still the characteristic form of tlie 

 chalceclonic crystallization is so minute that it is often very dif- 

 ficult to determine whether that which we are looking at 

 through the microscope is dimpled or mammillated, seeing that 

 the circles or little monticules which represent this are seldom 

 more than l-3000th of an inch in diameter. Frequently the 

 botr>"oidal surface is distinct ; as frequently also the little mam- 

 milliform projections are surroimded by rings ; and not unfre- 

 quently there aj^pears to be a dimple in the centre. But such 

 diflferences are of little moment if we remember that the sm- 

 face of these spicules, originally as smooth as glass, is now 

 rendered more or less uneven by the forms of crystallization 

 presented by chalcedony, and that this character distinctly 

 marks the difference between the organic and inorganic par- 

 ticles of which the deposit is composed. With this exception, 

 the spicules are but '' pseudomorphs," to use a mineralogical 

 term, of what they Avere in the living animal, where they were 

 produced. 



Nor should we forget the effect of the " solvent influence " 

 to which I have alluded, seeing that this also may have acted 

 at one time in one and at another in another way during the 

 transformation of the atomic constitution of the spicule, thus, 

 under certain circumstances, eroding the surface which received 

 an additional chalcedonic layer inider others, — and hence, as re- 

 gards erosion, the "reticulated" aspect noticed by Mr. Parfitt 

 in his excellent paper {l. c), which on the surface of some of 

 the spicules is so marked as to indicate that in this way many 

 may have altogether disappeared. This, too, may partly ac- 

 count for the apparently entire absence, above stated, of the 

 minuter and more delicate spicules which existing species, 

 almost identical with the fossil ones, as will be hereafter seen, 

 show us must have been present in the sponges to which they 

 originally and respectively belonged. 



Be this as- it may, the coarser features alone of the spicules 

 remain ; and so far altered is their original smooth surface by 

 erosion or the presence of the botryoidal form of chalcedony, 

 that not only is there an absence of the minuter and delicate 

 spicules, but also of all the minute sj)ines, tubercles, and other 

 markings which, in many instances, more or less cover and 

 characterize the large spicules of existing species, and thus 

 may be inferred to have equally covered and characterized 

 many of the fossil ones. 



As above stated, out of the seventy-nine illustrations there are 

 three only, viz. figs. 7, 8, and d, which are not representative of 

 the spicules in the greensand ; and these have been copied from 

 Schmidt and Du Bocage respectively, not less to illustrate the 



