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. [Sidenote: FOSSIL INFUSORIA OF BARBADOES.]