BEKSTED — ON THE GEOLOGY OF MAIDSTONE. 
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shape and appearance very like an alcjonium. Some of the branches 
of these fossils radiate from the central mass to a distance of 18 inches 
(fig. 7), each branch ending with a thick knob, and I have seen as 
many as twenty or thirty springing from a single stem. In some 
instances the sponges are prostrate, in others erect, as in fig. 4, plate 
xix. 
It is very rarely that shells are found in the stone of this group ; oc- 
casionally a detached valve of a terebratula or of an oyster is seen. 
The associated beds of hassock are very soft, and full of casts of 
cylindrical stems ; the lowermost stone-bed is remarkably cavernous, 
from the hassock below it being washed away, leaving the siliceous no- 
dules standing out. 
In 1846 I discovered in these beds a saurian bone bearing a great 
resemblance to the tibia of the Iguanodon ; but as only one end was 
preserved, I cannot come to any certain determination of it. 
A second group of limestone now follows, of a light colour, and as- 
sociated with hassock of a very good quality for building-purposes. 
The first bed is sometimes 3 feet in thickness, and the stone excel- 
lent for working up. The hassock ranges from 2 to 3 feet in thick- 
ness, and is of a coarse texture. No traces of stem-casts have been 
met with in this group. One of the most remarkable of the Lower 
Greensand spongeous organisms is, however, so characteristic of it, 
that in the course of twenty years' collecting I have not met with 
this form in any of the other strata. During that period I have col- 
lected a large number of specimens, showing a great variety of form 
and condition, it being only from a very numerous and well-selected 
series that a true conclusion as to their habits of growth and nature 
can be drawn. These fossils are, I believe, a species of Siphonia. The 
accompanying figures (PI. xvii. and xviii.) give numerous details of 
the principal and the subsidiary parts. It appears to have had a cylin- 
drical stem, with lobes at intervals in the course of its direction ; the 
lobes bulging out on one side more than on the other, and varying 
much in size. They are generally rougher or more covered with pa- 
pillae than the stem. Four, and even more, of these lobes have been 
seen on a single stem, the apex or top of which general consists of a 
nodule of siliceous sand, even when these sponges are embedded in 
the limestone rock. In the accompanying plate (PI. xvii.) the head 
is composed of siliceous sand, the stem and lower lobe of calcareous 
limestone of similar texture to the surrounding matrix. These con- 
ditions were so common, that I was led to examine the structure of 
the siliceous heads, and found that sponge-spiculae were much more 
abundant in them than in the stem and lower lobes, which invariably 
partake more or less of the properties of the surrounding stone. 
These sandy or cherty heads or nodules sometimes contain a con- 
tinuation of the stem and one of its lobes as a core within them. In 
some specimens only an ordinary portion of the stem passes through ; 
in others, these siliceous excrescences are permeated by numerous 
smaller branches ramifying through them. These nodules are full of 
sponge-spiculse, which are especially abundant round the stems and 
