Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/108

64 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 4, Ventriculite-structure, the skeleton is considered to be composed. On examining one of the above crosses, it is seen that its radii start from their common centre as very fine filaments indeed; but after pursuing their course for a quarter of their entire length, they undergo a sudden enlargement in diameter, and continue uniformly of this increased size to their terminal anastomosis. At the point of rather abrupt change in the diameter of the radius two other fibres are given off, one on each side, which pass backwards at an angle of about 45°, to join similar fibres similarly passing from the other arms of the cross; in this manner, about the centre of the cross under consideration, is formed a rough square, which represents the octahedral stays in section (fig. 1). This description applies equally to the sections figured of Toulmin Smith's Ventriculites in flint, and to those of the Cambridge forms in phosphatic material; and consequently, according to the admitted diagnosis of the family, there can be no doubt that our Cambridge fossils must be included in the same group with those of the Chalk, whilst a detailed examination has enabled me to identify, from our formation, forms belonging to some four of T. Smith's species; and there are others in my possession which I hope to determine when time allows.

Modes of Fossilization. — The fibre of our Cambridge Ventriculites presents itself under four different conditions of preservation.

(i.) Simple opaque fibres, of which the distal radial fibres are from 1/1250" to 1/1000" in diameter. Generally insoluble in hydrochloric acid.

(ii.) The simple fibre of No. i. is frequently surrounded by transparent colourless walls, from 1/3000" to 1/2000" in thickness, and generally well defined on their inner and outer circumferences. They give no colours with polarized light, but shine brightly on the dark ground produced by crossed prisms. Under a high power they seem to be made up of a number of crystals placed perpendicularly to the surface on which they are set. In hydrochloric acid they dissolve, but not so readily as the surrounding coprolite. The opaque fibre they enclose is next to insoluble in hydrochloric acid; it remains as an opaque yellowish-brown rod after the colourless walls have been altogether dissolved away.

(iii.) The opaque fibre of No. ii. has entirely disappeared, leaving only the colourless walls which surrounded it; these now form a cylindrical tube, measuring usually from 1/600" to 1/400" in diameter in the distal radii, and having a central hollow cavity of from 1/1250" to 1/60" diameter (fig. 1).

(iv.) The colourless walls and opaque fibre have both disappeared, and their place is occupied by a finely granular material, appearing in section as a broad band, of variable diameter, frequently from 1/500" to 1/400" across, generally of a brown colour, but sometimes pale green from infiltrated glauconite. With polarized light it appears strongly luminous on the dark ground produced by crossed prisms. In hydrochloric acid it is far less soluble than the adjacent coprolite.

In some sections of the Cambridge Ventriculites, simple opaque