188 ' Mr. BowERBANK on the Siliceous Bodies of 



however different in character the sponge or coral enclosed within the flint might 

 have been ; the spicula appearing to have no reference whatever to the bodies 

 thus enclosed, none of them being found upon the included body, but only upon 

 the case. When microscopically examined, the interior surface of these flint-cases 

 presented in every respect the same appearance as that exhibited by the under 

 surface of the tabular flints ; and there are generally fragments of minute corals, 

 small TerehratulcB and other shells attached to the inner surface ; and in the same 

 case there are frequently two, and sometimes three, different species of coralline or 

 spongeous bodies ; thus proving that these beautiful Wiltshire specimens are merely 

 extraneous bodies which have been built over and enveloped by the common coat- 

 ing sponge of the chalk. Many of the contained bodies also present evidence of 

 their having been exposed some time previous to being thus enveloped, as we fre- 

 quently find upon their surfaces patches of Flustra, which are in the habit of seat- 

 ing themselves upon dead or inorganic matter. Thin sections of the cases of the 

 Wiltshire fossils exhibit the characteristic Foraminiferoe and spicula, as well as the 

 common species of Xanthidia. 



It may be objected to this mode of accounting for the envelopment of sponges 

 and corals by the common casing sponge of the chalk, that the enclosed bodies 

 are usually almost in the centre of the flint, which would not be the case, if they 

 had been dead specimens lying at the bottom of the ocean at the time they were 

 built over ; but in reply to this it must be remembered, that although dead speci- 

 mens, they most probably retained the position in which they lived until the ex- 

 traneous mass surrounding them might alter their centre of gravity, when they 

 would fall on one side, and their bases would then, in all probability, be built over 

 in the same manner as the remainder of the mass. In many of these bodies, also, 

 there are portions of the enveloped sponge or coral which have never been invested, 

 and in numerous instances there are orifices remaining in the flint-cases through 

 which the roots of the enclosed body have passed out, and which were probably 

 preserved from being built over, either by being buried in the soft mud, or by 

 being attached at the bottom of the ocean. 



The tubuli of the flint-sponge are very minute : an average of the measurements 

 of four well-defined tubes upon the surface of a common flint gave the nVs^h of 

 an inch as the diameter ; the smallest of these was the i-gVrth, and the largest the 

 g^th of an inch in diameter. In the cast of an Ananchytes, five which were 

 measured gave an average of the 6-5-211^ of ^^ mc\iL ; the smallest of these being the 

 yygth, and the largest the 5-1-oth of an inch in diameter. In the cast of a Galerites 

 albogalerus, in which they were in a very beautiful state of development, a mea- 

 surement of seven gave the -girth as the average ; the largest being the yrgnd, and 



