The most remarkable forms are disciform frustules, having their surfaces elaborately ornamented with hexagonal spots disposed in curves, and bearing some resemblance to the engine-turned case of a watch. [Lign. 7, fig. 2], is a small segment of a disc, very highly magnified. These frustules vary in size from 1/100 to 1/1000 of an inch in diameter; they are named Coscinodiscus (sieve-like disc), and there are several species: one less richly sculptured, C. patina, is figured [Lign. 7, fig. 6]. Circular bodies, with five or six lines radiating from the centre to the circumference, like the spokes of a wheel, hence named Actinocyclus ([Lign. 7, figs. 4, 5]), and spicules of Sponges, are also abundant.
Lign. 7. Microphytes[69] from the Richmond-earth; highly magnified.
Tertiary. Virginia.
| Fig. | 1.— | Navicula. 1a. Side view. |
| 2.— | Coscinodiscus radiatus; a portion of the circular shield. | |
| 3.— | Galionella sulcata; the upper figure shows the transverse face of one of the frustules. 3a.—Three united cells viewed laterally. | |
| 4, 5.— | Actinocyclus. Two species. | |
| 6.— | Coscinodiscus patina; transverse view. 6a. Lateral view. |
[69] As the term Infusorial-earth must be abandoned, it will be convenient to substitute a name simply expressive of the nature of the most abundant organisms that enter into the composition of these deposits: that of Microphyta, or Microphytes, (from μικρος, mikros, small, and φυτον, phyton, a plant), signifying very minute vegetables, may perhaps be admissible: in this sense the word microphytal is employed in these pages.
When a few grains of the marl are prepared, and mounted on a glass, almost all these varieties will be manifest, so largely is this earth composed of organic structures; in fact, very few inorganic particles are intermixed, the merest pellicle left by the evaporation of a drop of water in which some of the marl has been mixed, teeming with the most beautiful structures.
At Petersburg, in Virginia, a sandy marl occurs, interstratified with deposits which, from their shells, are referred to the older tertiary formations. Probably this marl is a continuation of that of Richmond, but it is full of many new forms, associated with those common in the earth of the latter locality.[70]
[70] Dr. Bailey, with great liberality, has so amply supplied myself and other observers with specimens of these deposits for examination, that the fossils above described are familiar to all British microscopists. Figures of many of those organisms are given in the American Journal of Science.
It is an interesting fact, (first observed by Mr. Hamlin Lee,) that the common Scallop (Pecten maximus), as well as the Barnacle (Balanus), feed on diatomaceæ, and their stomachs generally contain numerous cases of Coscinodisci, Dichtyochi, Actinocycli, &c.: a slide prepared and mounted with the contents of the stomachs of these mollusks, presents an assemblage of forms identical with those found in the tertiary earths of Virginia.[71]