[208] The pebbles represented in Pict. Atlas, pl. xlv. fig. 5, 12, are specimens of this kind. [Sidenote: 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. [Illustration: Lign. 73. Siphoniæ, from the Greensand; _nat._ _Wilts, and Isle of Wight._ Fig. 1.--Siphonia pyriformis; the body or upper part. 2.--Vertical section of the same, showing the internal structure, and the central cavity. 3.--Specimen of S. pyriformis in a young state. 4.--Siphonia (_Polypothecia_, of Miss Benett,) lobata; Firestone or Upper Greensand, Warminster. 5.--The lower part of the stem, and radicles, of S. pyriformis. ] A group of Sponges from the Upper Greensand, near Warminster, figured and described by the late Miss Etheldred Benett,[210] under the name of Polypothecia, comprises several forms that are allied in structure to the Siphoniæ. These fossils present considerable diversity of shape; one of the lobed forms is delineated in _Lign. 73, fig. 4_: and a branched species in _Lign. 74_. Upon breaking the stem of one of these zoophytes transversely, sections of parallel longitudinal tubes like those in the Siphoniæ are exhibited. [210] An elegant Memoir on the Wiltshire Fossils, by this accomplished lady, is published in Sir R. C. Hoare's "Wiltshire." [Illustration: Lign. 74. Polypothecia dichotoma. _Upper Greensand, Warminster, Wilts._]