Spiniferites (Xanthidium, Ehrenb.). [Lign. 77.]—I propose to describe in this place those elegant and very minute bodies, that occur in great numbers in the chalk and flint, and which, on the authority of M. Ehrenberg, were regarded as identical with the siliceous frustules of the genus of fresh-water Desmidiæ, named Xanthidium[218] (ante, [p. 91].) Later and more correct observations have proved that the fossils under consideration entirely differ from their supposed homologues; their original substance not being siliceous, but flexible and membranous; and that instead of being spores of algæ, they are probably the gemmules either of porifera, or of polypifera.
[218] Several recent species of Xanthidium are figured in Plate IV. of this volume.
To avoid the perpetuation of the error by the retention of the botanical name of a recent genus of plants, for fossils whose vegetable origin is very problematical, and which are entirely distinct from their supposed analogues, I would substitute that of Spiniferites,[219] a term simply expressive of the general aspect of these bodies; that of a globe or sphere beset with spines. The appearance of these fossils will be understood by the examination of a group discovered by Mr. Reginald Neville Mantell, in a fragment chipped off from a flint pebble; and I will describe the mode by which these minute objects were detected, as it offers a good practical lesson for the young investigator.
[219] From spina, a spine, and fero, to bear.
The chip of flint is represented, of the natural size, in [Lign. 77, fig. 1]; it was immersed in oil of turpentine for a short time, and then placed on a piece of glass, and examined with a moderate power, by transmitted light, the turpentine having rendered the translucent flint almost as transparent as glass; this appearance is shown in fig. 2; the organisms here represented are from 1/300 to 1/500 of an inch in diameter. The half-inch object-glass was next employed, and fig. 3 gives the result. The quarter-inch object-glass, and a corresponding eye-piece, were then substituted, and by the adaptation of a camera lucida, figs. 4, 5, and 6, were delineated. As fig. 5 proved to be a new species, it was named after its discoverer.
The specimens in flint, when rendered transparent and viewed by transmitted light under a high power, as shown in Lign. [78], and [79], appear as hollow globular bodies, beset with spinous processes, which in most species are fimbriated at the extremities. There is considerable variety in the form and length of the spines. In S. Reginaldi, these appendages are numerous, regular, short, and relatively thick: in an elegant species discovered by the Rev. J. B. Reade ([Lign. 79]) they are long and palmated: in other kinds they are of intermediate size and proportions.[220]
[220] Excellent figures of several species are given in a Memoir by H. H. White, Esq. of Clapham, in the Trans. Microscopical Society, vol. i. p. 77.
| Lign. 78. Spiniferites Reginaldi. (Magnified 500 diameters.) | Lign. 79. Spiniferites palmatus; in flint. (× 500 diameters.) |
The apparently torn and collapsed state of the body and arms of some examples first led me to doubt the siliceous nature of the original substance; and on my discovery of the soft parts of foraminifera in flint and chalk, Mr. Deane undertook to search for the so-called Xanthidia in chalk, that these bodies might be subjected to chemical analysis,[221] Mr. Deane succeeded in detecting all the usual species in the Dover chalk, by digesting some chalk in dilute hydrochloric acid, and mounting the residue in Canada balsam. In this state the shape of the body is that of a depressed sphere; many of the specimens appear to have a circular opening, and the arms or spines to be closed at the extremities. Upon pressure under water between two pieces of glass, they were torn asunder as a horny or cartilaginous substance would be, and the spines in contact with the glass were bent. Some after maceration in water several weeks became flaccid; a proof that they are not siliceous.[222]
[221] A torn and apparently shrunken specimen from chalk, is represented in my paper on Foraminifera; Philos. Trans. 1846, p. 465.