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drical contorted canals, which from their uniformity and minuteness 
of diameter, Mr. Bowerbank considered to be the incurrent canals of 
the sponge; and other orifices of greater diameter, to be the excur- 
rent. Very frequently, when little of the reticulated substance of the 
sponge remains, its former presence, the author says, is indicated by the 
siliceous matter resembling a congeries of gelatinous globules, mould- 
ed by the tissue amid which it was deposited ; and the globules, when 
traced to the edges of the patches of spongeous texture, were found 
to agree in size and form with the orifices of the supposed incurrent 
canals. In cases where no traces of the sponge can be detected, Mr. 
Bowerbank thinks, that the mode in which the spicula, foraminifera 
and other extraneous matters are dispersed equally in all parts, and 
not precipitated to one portion of the flint, indicates that the organ- 
ized tissue in which they were entangled, retained its form and tex- 
ture sufficiently long to allow of the fossilization of these remains in 
their original places ; and that the nature and position of these bodies 
strongly indicated the former spongeous nature of the flint. 
When tlie chalk is carefully washed from the exterior of a flint, 
and a portion examined as an opake object with a power of about 
fifty linear, it exhibits a peculiar saccharine appearance, with deep 
circular excavations, having fragments of extraneous matters partly 
imbedded or adhering to them. If the surface be further cleansed by 
immersion in diluted muriatic acid, till effervescence ceases, spicula 
may be detected on the sides of the deep circular cavities; and if, 
again, a piece a quarter of an inch in diameter, presenting the rough- 
est aspect, be examined under a power of 120 linear, illuminated by a 
Leiberkuhn, the surface, under favourable circumstances, will pre- 
sent a complex mass of small, contorted tubuli, occasionally fur- 
nished at the apex with a minute perforation. 
The structure and other characters of the tabular flints are stated 
to accord perfectly with those of the nodular masses, except that the 
under surface has a still more marked spongeous aspect, and that spi- 
cula and foraminifera are more abundant. The absence of any ap- 
parent base or point of attachment in the great mass of nodular chalk 
flints, the author says (considering them undoubtedly of spongeous 
origin) may be accounted for by supposing that the gemmule was 
originally attached to some minute fragment of a shell or other sub- 
stance, and that its further development took place while recumbent 
on the mud or silt. 
The perpendicular and oblique veins of flint between Brighton 
and Rottingdean, are reported to present exactly the same internal 
characters as the tabular and nodular flints, and to agree externally 
with the former. The occasional existence of a fissure filled with 
chalk, in the centre of the vertical layers, Mr. Bowerbank conceives, 
may indicate that the sponge had grown from the two sides of the 
crevices, but had not in all places been able to unite. The sides of 
these flint veins are not studded with foraminifera in a manner simi- 
lar to that of the tabular horizontal layers. 
Mr. Bowerbank next examined the flint with which Echinites and 
shells of the chalk are often entirely or partially filled and enveloped, 
