Choanites Königi, [Lign. 75.]—The zoophyte which has given rise to the fossils I have distinguished by the name of Choanites,[213] is of a spheroidal or sub-ovate form, and appears to have been composed of a softer tissue than the ordinary sponges. It has a central cavity, and was fixed at the base by long rootlets: its mass is traversed by numerous tubes or channels, which open on the inner surface of the cavity; it differs from Siphonia in not having a stem composed of bundles of tubes, and probably also in its constituent substance. Among the Sussex and Wiltshire chalk-flints specimens of this zoophyte are very common; they are easily recognized by the peculiar markings produced by the silicified tubes that radiate from the centre, as seen in [Lign. 75, fig. 4]. The semi-diaphanous pebbles on the Sussex coast, more frequently contain Choanites than any other zoophytes. From the beautiful and varied markings observable in the polished sections, they are in great request for brooches, and are termed petrified sea-animal flowers[214] by the lapidaries; among the shingle on the sea-shore at Bognor, Worthing, and other places, very fine examples may be collected.
[213] Foss. S. D. p. 178.
[214] From the supposition that the original was an Actinia, or Sea-Anemone. A coloured vertical section of a pebble of this kind is figured in my "Thoughts on a Pebble," Eighth Edition, pl. ii. See coloured figures of Choanites in Pict. Atlas, pl. xlii. fig. 1, 9, and 10; pl. xliv. fig. 8; and pl. xlv. fig. 10.
Lign. 75. Choanites Königi. (G. A. M.)
Chalk, Lewes.
| Fig. | 1.— | Transverse section of a siliceous specimen. |
| 2.— | Upper portion of a Choanite, in chalk, showing the opening of the central cavity at a. | |
| 3.— | Vertical section of a Choanite, in flint, exposing a section of the mass, and canals passing obliquely from the central cavity, through the substance. | |
| 4.— | Choanite in flint; the usual appearance of these fossils. | |
| 5.— | Various kinds of Spicula of fossil sponges; magnified. |
Lign. 75, fig. 4, represents the usual appearance of a flint deriving its form from a Choanite; fig. 2, is the upper part of a Choanite preserved in chalk, and richly coloured by iron; the opening at the summit, a, is the orifice of the central cylindrical cavity, which is in this instance filled up by chalk, but in flint specimens, with silex of a different colour to that of the surrounding mass. If fig. 2, were placed on the top of fig. 4, the general shape of the original zoophyte would be represented. The opening at the base of fig. 4, marks the spot whence the processes of attachment proceeded. The vertical section of a flint, similar to fig. 4, is shown at fig. 3; and in this example are seen the central cavity, and a section of the substance of the zoophyte, which is traversed by numerous tubes, that ramify through the mass of which the body was composed, and terminate in openings on the inner wall of the central cavity, or sac. A transverse section of a similar flint is delineated in fig. 1; the central white spot indicates the sac filled with flint, and the tubes are seen radiating from it through the mass; under a powerful lens the interstitial structure appears to be granular rather than porous. The perfect transparency of the body when silicified, and the rich tints it has acquired from metallic solutions, and the compressed state in which it is often found, seem to indicate that the original mass was a soft gelatinous substance, like that of the Actinia, strengthened by spicula; for numerous tri-radiate spines, like those on the left-hand of [fig. 5, Lign. 75], occur occasionally in chalk specimens.
In many Choanites, which differ in no other respect from the present species, vertical sections show on each side the central cavity, large oval spots, that are sections of a canal which traverses the entire mass, proceeding from the base to the summit, in a spiral coil around the central cavity. This structure was first detected by Mr. Cunnington. Mr. Woodward thinks this spiral tube is common to all the Choanites, and constitutes a generic character; but so many examples have passed under my examination in which no traces of such a canal are perceptible, that it may be a specific difference.