the Chalk, Greensands and Oolites. 189 



the smallest the gioth of an inch in diameter. The tubuli in this specimen were 

 hollow ; and one of them, which had been broken at a right angle to its axis, gave 

 the following measurements : external diameter gi^th, internal diameter j-^g-and of 

 an inch ; thickness of the substance of the tube 4T6Tth of an inch. In their size and 

 proportion the tubes agree very nearly with those of the recent sponges. The 

 average diameter of the fibre of the common sponge of commerce is about the 

 Yi^th of an inch, and in the sponge from New South Wales the diameter varies 

 from the grrst of an inch to the 47-3 nd. The spicula are large in proportion to the 

 other parts of the tissue, very frequently exceeding the eighth or tenth of an inch 

 in length. They are thinly scattered, and usually acicular in their form ; but some- 

 times they are forked or branched, but this is comparatively a rare character. They 

 do not occur in fasciculi, as in Halichondria, but are usually solitary, or a few are 

 grouped together. They are mostly of an elongated, spindle-formed shape and 

 somewhat curved, as represented in fig. 3, PI. XVIII., which exhibits them in 

 their natural position, on the interior surface of a fiint from Wiltshire ; or in fig. 4, 

 PI. XVIII., which gives a series of the largest of them, separated from the white 

 friable siliceous matter from the interior of one of the Wiltshire flints. 



The results arising from the examination of the flinty bodies of the chalk in- 

 duced me to believe that the cherts of the greensand formations and of the oolites 

 would probably prove to be of similar origin. I therefore examined thin slices 

 of specimens of chert from the upper greensand pits of Fovant in Wiltshire, in the 

 manner pursued in the investigation of the flints ; and I found, as I expected, the 

 result to be of a similar description ; but in this substance the spongeous fibre is 

 of a much coarser texture than in that of the chalk-flint, and the interstices of the 

 network very much larger in proportion. The imbedded extraneous matters are 

 also of a larger description, such as small fragments of apparently very fine branched 

 vegetables, etc., in addition to Xanthidia and other small organic bodies. The 

 form and mode of ramification of the tubular structure approaches nearer to those 

 of the fibre of the common Mediterranean sponge than in the flint spongite. The 

 tubes are seen dispersed in about an equal proportion through the whole mass of 

 the chert. They are very pellucid, and would probably escape observation, if it 

 were not that their surfaces are covered w^ith a short downy-looking structure, 

 which gives them very much the appearance of minute portions of a very trans- 

 parent Fucus, when viewed as a transparent object, with a microscopic power of 

 fifty linear. Sometimes, but very rarely, portions of the tubes remain hollow, and 

 they then display a very distinct and characteristic appearance, such as that ex- 

 hibited by fig. 1, PI. XIX., which represents a portion of the tubes under these 

 circumstances, as they appear when viewed as opaque objects, with the aid of a 

 leiberkuhn. The average measurement of five of these tubes gave a diameter of 



