Diatomaceæ, Recent and Fossil.
Fig. 298.—Fossil Diatoms from Springfield (Barbadoes).
1, Achnanthidium; 2, Diatoma vulgare, side view and front view; 3, Biddulphia; 4, 5, 6, 7, Amphitetias antediluviana, front view, with globular and oval forms; Gomphonema elongatum and capitatum.
Fossilised Diatomaceæ.—Dr. Gregory was of opinion that a large number of diatoms separated into species are only transition forms, and more extended observations have proved that form and outline are not always to be trusted in this matter. Species-making is a modern invention, and can hardly apply to the indestructible fossilised forms of the frustules of Diatomaceæ, with their beautiful sculpturings and geometrical constructions, which have not been materially changed since they were first deposited. Startling and almost incredible as the assertion may appear to some, it is none the less a fact established beyond all question, that some of the most gigantic mountain-ranges, as the mighty Andes, towering into space 25,250 feet above the level of the sea, their base occupying vast areas of land; as also massive limestone rocks; the sand that covers boundless deserts; and the soil of many wide-extended plains, are each and all principally composed of Diatomaceæ. And, as Dr. Buckland once observed: “The remains of such minute animals have added much more to the mass of materials which compose the exterior crust of the globe than the bones of elephants, hippopotami, and whales.”
In 1841 the late Mr. Sollitt, of Hull, discovered the beautiful longitudinal and transverse striæ (markings) on the Pleurosigma hippocampus. A curved graceful line runs down the shell, in the centre of which is an expanded oval opening. Near to the central opening the dots elongate crossways, presenting the appearance of small short bands.
In the vicinity of this town many interesting varieties of Diatomaceæ have been found, the beauty of the varied forms of which are constantly under investigation; at the same time some of them are highly useful, as forming that class of test objects which are better calculated than many others for determining the excellence and powers of certain objectives. Mr. Sollitt carefully measured the markings on some of the frustules and found they ranged between the 1⁄30000th and 1⁄130000th of an inch; the Pleurosigma strigilis having the strongest markings, and the Pleurosigma acus the finest.
Mr. J. D. Sollitt not only first proposed their use, but he also furnished the measurements of the lines of the several members of this family, as follows:—
Amphipleura pellucida, or Acus, 130,000 in the inch, cross lines.
" sigmoidea, 70,000 in the inch.
Navicula rhomboides, 111,000 in the inch, cross lines.
Pleurosigma fasciola, fine shell, 86,000 in the inch, cross lines.
"" strong shell, 64,000 in the inch, cross lines.
" strigosum, 72,000 in the inch, diagonal lines.
" angulatum, 51,000 in the inch, diagonal lines.
" quadratum, 50,000 in the inch, diagonal lines.
" Spencerii, 50,000 in the inch, cross lines.
" attenuatum, 42,000 in the inch, cross lines.
" Balticum, 40,000 in the inch, cross lines.
" formosum, 32,000 in the inch, diagonal lines.
" strigilis, 30,000 in the inch, cross lines.
PLATE XII.
MICRO-PHOTOGRAPH OF TEST DIATOMS.