230 THE MEDALS OF CREATION. Cuap. VII. | 
expanded ; not fibrous and root-like, as in the spongites of — 
the chalk previously described. 
Among the shingle at Brighton, Margate, Dover, Isle of — 
Wight, &c. pebbles containing fossil sponges may frequently — 
be discovered. When the flint nodule has been broken, and ~ 
the calcareous particles of the inclosed zoophytes are washed — 
away by the action of the waves, a delicate silicified tissue — 
remains.* Many of the large solid pebbles, are portions of 
silicified sponges, and polished specimens are beautiful objects 
under the microscope. 
SrpHonia. Lign. 73.—'These fossil porifera are readily — 
distinguished from those which have engaged our attention — 
by their more symmetrical structure. The body of the ~ 
zoophyte is a mass of dense porous tissue, of a pyriform or — 
bulbous shape, supported by a slender stem fixed at the base — 
by rootlets. The stem is composed of very fine parallel longi- 
tudinal tubes, which extend to a series of canals that traverse 
the mass, and terminate in openings on the surface of a 
shallow central cavity, as shown in the section, jig. 2, Lign.73. — 
The characters of this genus are well exemplified in a com- 
mon species of the Greensand (S. pyriformis, Lign. 73), 
described by the late Mr. Webster, from specimens collected — 
in the Isle of Wight, where it occurs in profusion, near 
Ventnor, and the Western lines. This zoophyte is pyriform, 
(Lign. 73, fig. 1,) and has a shallow cylindrical cavity, sup- — 
ported upon a long slender stem, the base of which is fixed — 
by root-like processes (fig. 5) ; the transverse fracture shows 
a section of the longitudinal tubes. This species has been 
found in numerous localities of the Greensand, and also in 
the Firestone or malm-rock.t 
The Portland limestone contains numerous remains of a— 
* The pebbles represented in Pict. Atlas, pl. xlv. fig. 5, 12, are © 
specimens of this kind. 
+ Dr. Fitton’s figures, Geol. Trans. vol. ii. pl. xv.a, are very beau- — 
tiful and accurate. 
