Fenestrella.--Cells very small, indistinct externally, with small prominent openings; polyparium stony, fixed at the base, composed of branches, which inosculate by growth, and form a cup. Numerous delicate corals, formerly arranged as _Reteporæ_, occur in the Silurian rocks, and have been placed in this genus by Mr. Lonsdale. (_Sil. Syst._ p. 677.) Petalæpora pulchella.[254] _Lign. 69, fig. 1._--This beautiful cretaceous coral is "tubular, free except at the base; framework composed of vertical laminæ, with an intermediate foraminated structure; apertures to the tubular cavities distributed over the surface; exterior varying with age." It has slender round dichotomous branches, and the polyparium when entire must have formed an elegant plexus of coral. A layer an inch thick, full of branches of this zoophyte, is exposed on the face of the chalk cliffs, near Dover; and beautiful masses, several inches square, made up of this coral, Idmonea and Pustulopora, may be obtained. The microscopic specimen figured in _Lign. 69_, was obtained with many other corals by washing chalk with a brush, and examining the detritus deposited. [254] Mr. Lonsdale. Dixon's Fossils, p. 285. Pustulopora. _Lign. 89, fig. 8._--Another very common tubular branched coral of the Dover chalk; the tubes are cylindrical, their apertures are arranged in annular or spiral rows, and slightly projecting, giving a pustulous appearance to the stem and branches. Specimens covering a piece of chalk six or eight inches wide, and a foot long, have been discovered. The example figured is a very minute branch. Homœsolen ramulosus.[255] _Lign. 89, figs. 9, 11._--This delicate branched coral is formed of large and small tubes variously intermingled, both inclined in the same direction, partially visible on the surface, or wholly concealed, limited to one side of the coral; mouths simple tubular extremities; back without pores, composed of a continuous lamina.[256] [255] Homœsolen, from ομοιος, similar; and σωλην, a tube. [256] Mr. Lonsdale, in Dixon's Fossils, p. 307, tab. xviii. B. figs, 3, 4, 5. The elegant coral, _fig. 11, Lign. 89_, is thus named by Mr. Lonsdale; it resembles his _fig. 4_. The fossil, _fig. 9, Lign. 89_, though very different in its branching, and in the surface, which is covered with pores, is evidently identical with _fig. 3_ of Mr. Lonsdale, which he refers to the same species. Idmonea, _Lign. 89, fig. 6._--In this elegant coral the polyparium is calcareous, branched, porous; the cells distinct, prominent, arranged in single rows, more or less inclined, on each side a median line on the inner face only. The genus is extinct.