84 



measuring as before from the ends of the opposite tentacula, is from 

 TTs to ytw of an inch. 



I may here observe that the recent Xanthidia, discovered alive in 

 w ater which was sent by Professor Bailey to Mr. E. Quekett, from a 

 pool or lake at West Point, near New York, in America, and a few 

 drops of which were given to me by Dr. Mantell and the Rev. Charles 

 Pritchard, of Clapham, resembled closely the fossil species of the Tu- 

 bifera in the tubiform character of the tentacula ; but in reference to 

 the terminations of those tentacula (being trifid) they most resembled 

 one variety of the species ramosum. The apparent tentacula of the 

 recent Xanthidia were from nine to fourteen in number, measuring in 

 extreme diameter ^^ of an inch. I had the gratification of seeing 

 one of these recent specimens move its tentacula, and I preserved it 

 in its collapsed state. I have another recent Xanthidium from the 

 same source, which, to an inexperienced observer, could scarcely be 

 distinguished from one in the fossil state. 



The discovery of these recent specimens proves, beyond all doubt, 

 that these various fossil species are animal and not vegetable substan- 

 ces ; and judging merely from the appearance of these fossil species, 

 we might conclude that they were living at the moment when, by some 

 wonderful agency, they and the surrounding fluid in which they were 

 living became converted into one solid mass of silex, assuming their 

 present form of nodules of flint. I have, perhaps, unphilosophically 

 said at the moment when, by some wonderful agency, these floating 

 animalcules and the circumambient fluid which sustained them, be- 

 came converted into one solid mass of silex ; for I believe the forma- 

 tion of these chalk flints is considered among the learned in Geology 

 a problem by no means of easy solution. That flint may be held in 

 solution, in considerable quantity, in some fluids, I believe is certain ; 

 but by what process, by what agency, during the Cretaceous period, 

 the flint nodules, extending in long parallel layers or veins in chalk 

 cliffs or otherwise, assumed their present form, I must leave to those 

 who, from their scientific acquirements, are more competent to form 

 an opinion on the subject. Whether, by a sudden transformation, the 

 surrounding fluid which contained these minute and delicately organ- 

 ized modifications of animal existence we have been describing, was, 

 with its numerous inhabitants, converted into its present condition of 

 flint; or whether, by the gradual deposit of minute particles of silex, 

 held in solution by the fluid which contained these minute animal- 

 cules, the continuous aggregation became nodules of flint, as we now 

 find thcin, in various chalk districts in England ; or whether, by some 



