194 THE MICROSCOPE. 



cumstances alone seem clearly to disprove the idea of their being 

 purely siliceous. The casts of the Polythalamia, portions of minute 

 crustaceans, &c. appeared also to be, like the Xanthidia, some modifi- 

 cation of organic matter ; and in the case of the Polythalamia, the 

 bodies are so perfectly preserved, that in some the lining membranes 

 of the shells are readily distinguishable. 



These investigations corroborate Mr. Kalfs, and also show that the 

 same fossil remains, so abundant in flint, are to be found in the beds of 

 chalk by which it is surrounded. 



Mr. Wilkinson, who examined some Xanthidia found in the Thames 

 mud and slime on piles and stones at Greenhithe, gives it as his 

 opinion that they are not siliceous, but of a horny nature, similar to 

 the wiry sponges, which Mr. Bowerbank describes as being very diffi- 

 cult to destroy without the action of fire. 



He also met with a peculiarity in a X. spinosum, which he had 

 never seen in any other species ; it was in a piece of a gun-flint. There 

 appeared, as it were, a groove or division round the circumference, 

 similar to that formed by two cups when placed on each other, so as 

 to make their rims or upper edges meet. There also seemed to be a 

 peculiarity in the arrangement of the tentacula : the tentacula of Xan- 

 thidia generally pass from the body in a direction perpendicular to its 

 centre, without any apparent fixed arrangement of position ; but in this 

 instance, both in the upper and lower portions, there were two circlets 

 of tentacula ; one placed round each extremity, and another round that 

 part where the specimen appeared to be separated. The size was the 

 325th part of an inch in diameter, and the number of tentacula about 

 twenty-five. 



The other fossil Infusoria, found most abundantly in the chalk and 

 flint of England, are the Rotalia or wheel-sliaped, and the Textularia or 

 woven-work animalcules ; the latter having the appearance of a cluster 

 of eggs in a pyramidical form, the largest being at the base, and lessen- 

 ing towards the apex. 



