These organisms are of exquisite structure, and comprise many species and genera. The most beautiful and abundant are the circular shields, termed Coscinodisci (sieve-like disks), which are elegant saucer-shaped cases, elaborately ornamented with hexagonal apertures disposed in curves, somewhat resembling the engine-turned sculpturing of a watch; these shells are from 1/1000 to 1/100 of an inch in diameter. A segment of one of these disks, highly magnified, is represented in [Lign. 18, fig. 2]. The body of the living animalcule was protected and enclosed by a pair of these concave shells, the perforations admitting of the exsertion of filaments or tentacula. This species of Coscinodiscus abounds in the present seas, and constitutes no inconsiderable proportion of the food of Pectens and other testaceous mollusca.[AX]
[AX] See 'Thoughts on Animalcules,' p. 103.
All the animalcules found in the Richmond earth are marine, and most of them belong to genera, and many to existing species; although the position of the American strata proves that they are referable to a period of immense antiquity.
In Germany, beds of a white infusorial earth, resembling magnesia in appearance, and termed Bergh-mehl, or fossil farina, occur at Bilin, and several other places: at San Fiora in Tuscany, near Egra in Bohemia, in the Bermudas, Barbadoes, &c., similar deposits have been discovered; all being composed of the shields of various kinds of animalcules. But I must not extend these remarks, and will only add a few observations on the infusorial earth of Barbadoes, which has but recently been brought under the notice of geologists by Sir Robert Schomburgk, and is especially interesting for the exquisite beauty and variety of its organisms, and the circumstances under which the deposit occurs.
FOSSIL INFUSORIA OF BARBADOES.
Barbadoes, an island of the West Indies, is about twelve miles in length from north to south, and consists of coral reefs, capped in one district by tertiary sandstones and limestones, which attain a height of 1200 feet above the sea. Over the rest of the island, coral reefs form the entire surface, which is divided by vertical walls of coral, some of them nearly 200 feet high, into six terraces, indicating as many periods of upheaval. In the lowest reef, Indian hatchets have been found twenty feet above high water mark; shewing that the last movement, at least, took place within the human period. The tertiary strata are more or less inclined, and in many places vertical, and contorted. Strata of marl, several hundred feet thick, predominate; and there are beds of bituminous coal, sandstone, clays, and ferruginous sands. Arenaceous limestone containing teeth of sharks, spines of echini, and shells, forms the summit of a hill nearly 1,000 feet high. The white marls abound in 300 species of the most beautiful siliceous infusoria; many are peculiar, others the same as occur in the Richmond earth, and some belong to recent species.[AY]
[AY] Sir R. H. Schomburgk: Brit. Assoc. 1847.
THE END.