190 Mr. BowERBANK on the Siliceous Bodies of 



3X4 th of an inch ; the largest being the areth, and the smallest the 4i^th of an 

 inch. 



The cherty nodules from the upper greensand of Shaftsbury presented appear- 

 ances exceedingly similar to those observed in the specimens from Fovant. The 

 black and semi-transparent mass of chert in the centre was surrounded by a coat- 

 ing nearly an inch in thickness, which had the appearance of agglutinated sand ; 

 and the outer surface of the coat assumed an eccentric tuberous form, very similar 

 to that of the chalk flints. 



Upon microscopically examining the structure of this coat, as an opaque object, 

 numerous contorted canals of various sizes were observed, and beautiful green spi- 

 cula were visible imbedded in considerable numbers in the mass. It would therefore 

 appear, that the same process went forward during the imbedment of this sponge, 

 which we have supposed to have taken place during the envelopment of the sponge 

 which originated the chalk flints ; but the sponge, in this instance, being of a 

 coarser and more open nature, it has been penetrated by the surrounding fine sandy 

 matter to a greater proportionate depth than in the case of the chalk flints. Two 

 casts of Spatangus in chert, from the upper greensand pits at Shaftsbury, presented 

 results analogous to those obtained by the examination of the flint casts from 

 the chalk ; the surfaces exhibiting numerous tubes and spicula, which corre- 

 sponded in size and appearance with those shown in the thin sections represented 

 by fig. 1, PL XIX., so that little doubt can exist of the formation of these casts 

 being similar to that of the chalk flints. 



Upon examining in the same manner a number of slices from a great variety of 

 masses of chert belonging to the greensand of Lyme Regis, nearly the same appear- 

 ances were presented as those afforded by the chert of the upper greensand of Fo- 

 vant. A group of tubuli are represented at fig. 2, PI. XIX. : and their size, mode 

 of disposition, as well as general character, so closely resemble those of the upper 

 greensand, that there is strong reason to believe the sponge to have been of the 

 same species, or very closely allied to it ; and it was only when they were examined 

 as transparent objects, with a power of 360 linear, that I could detect any evi- 

 dent diflference. Under these circumstances the tubes of the former present the 

 semblance of a mucous surface, with minute irregular particles of opaque matter 

 adhering to them, while the tubes of the latter exhibit an appearance as if minute 

 patches of a thin epidermal membrane had been partly detached from their sur- 

 face ; but this trifling variation in the appearance of the organic structure may 

 probably have been caused by a slight difference in the circumstances attending 

 their fossilization. The tubuli vary in their size to about the same extent as those 

 of the upper greensand spongite, but their average diameter is as nearly as pos- 

 sible the same. 



