| Fig. | 1.— | Tragos peziza. |
| 2.— | Probably Chenendopora in a young state. | |
| 3.— | Verticillipora anastomosans. | |
| 4.— | Species of Scyphia. | |
| 5.— | Tragos Faringdoniensis. |
Tragos. [Lign. 72, fig. 1.]—These turbinated sponges are readily distinguished from the preceding, by the relatively large oscula, or open papillæ, disposed irregularly on the inner surface; as shown in the specimen, fig. 1. Their tissue is dense and fibrous. The fossil represented by fig. 5, though named Tragos by collectors, appears to differ in the structure of the inner surface from the type of this genus: it is a remarkably beautiful species.[207]
[207] It may be convenient to distinguish it as T. Faringdoniensis.
The base in all these Greensand sponges is flat and 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.[208] Many of the large solid pebbles, are portions of silicified sponges, and polished specimens are beautiful objects under the microscope.
[208] The pebbles represented in Pict. Atlas, pl. xlv. fig. 5, 12, are specimens of this kind.
SIPHONIA.
Siphonia. [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 longitudinal 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, [fig. 2, Lign. 73]. The characters of this genus are well exemplified in a common 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, supported 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.[209]
[209] Dr. Fitton's figures, Geol. Trans. vol. ii. pl. xv. a, are very beautiful and accurate.
The Portland limestone contains numerous remains of a Siphonia closely resembling this species; and varied sections of its stems produce the white markings commonly observable on the slabs of pavements.