Polystomella.—In its general form this genus resembles the other nautiloid shells above described, but its structure differs essentially; for there are several apertures along the side of the shell, as well as the opening in the last segment. The cells are simple, and each is a single cavity. The figures and details of structure, given by Professor Williamson, must be referred to, for an insight into the organization of this beautiful and complicated type of Foraminifera.[319] One species of Polystomella is said to occur in the Chalk; I have not detected this genus in our cretaceous deposits. Recent species swarm in our seas; and may be easily obtained from the mud and sand on the shores at Brighton.[320]
[319] Trans. Microscop. Society, vol. ii.
[320] Mr. Poulton has specimens of the shells, and the bodies of the animals deprived of the shell, mounted for the microscope.
Lituola. [Lign. 111, fig. 3a.]—In a young state the shell is nautiloid, as in fig. 3b, 3d; but becomes produced by age, and assumes a crosier-like form, as in fig. 3a. The cells are filled with a porous testaceous tissue, as shown in figs. 3b, 3c; which also illustrate the foraminiferous character of the shells in this class of animals; for both the external testaceous covering, and the septa of the cells, exhibit perforations.[321]
[321] The perforations are omitted, by mistake, in the figure of the adult shell, fig. 3a.
Lign. 112. Spirolinites in Flint.
Chalk. Sussex.
(By the late Marquis of Northampton.)
The specimens are only sections of the shells Imbedded in flint, and seen as opaque objects with a lens of moderate power. The small figures denote the size of the originals.
SPIROLINA.
Spirolina. [Lign. 112.]—The general form resembles that of Lituola: the young shell being a discoidal, involute, and becoming produced by age; but the internal structure is different; the cells are simple cavities.