Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/109

1872.] SOLLAS — UPPER-GREENSAND VENTRICULITAE. 65 spherical bodies are found from 1/200" to 1/400" diameter, which agree in their mineral characters with the opaque yellowish-brown fibre of No. ii.

The Hexaradiate Elements. — The Ventriculite skeleton is formed by the regular anastomosis of a number of hexaradiate elements, each one of which consists of six radial fibres, apparently tubular, diverging from a common centre at right angles to each other. To the end of the first quarter of their course the fibres are of the most microscopic diameter; but they then suddenly expand, their diameter becomes many times greater, and continues so to the end of their entire course, where they open into the similar radii of surrounding elements. At their point of expansion they give off four fibres of a diameter of about 1/2500", which pass backwards at an angle of about 45° to join similar fibres given off by the other arms of the same element, so as to form a skeleton octahedron about the common centre (fig. 2). This is the typical structure of the hexaradiate

elements. As in the Vitrea, however, abnormalities occur, the elements sometimes becoming pseudo-heptaradiate (fig. 3), and sometimes apparently triradiate. The additional ray of the heptaradiates is prolonged merely from one of the octahedral fibres, and does not start from the common centre of the six normal rays. The triradiates also have nothing in common with the hexaradiate elements.

Combination of the Hexaradiate Elements. — Considering one of these elements, it will be seen that each of its four horizontal radii is freely continuous in the same straight line with a horizontal radius furnished to it by one of four other elements symmetrically disposed in the same plane around it; each of the vertical radii is continuous in the same way with a vertical radius derived from each of two other elements, one placed above and the other below it: all the

VOL. XXIX. — PART I. F