Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/122

78 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 18, The osculiform pits are grouped sometimes at the end of one of the wrinkled depressions above described; sometimes they are collected in sieve-like patches, or two or three may be noticed at the bottom of a wider-mouthed pit, while occasionally they are terminal, and frequently dispersed. It may be noticed that in the same specimen these pits occur of very various sizes, and that the striated grooves of their margins are never arranged in any multiple of four — an argument, if arguments are needed, against the alcyonioid origin of these forms.

(iii.) The puncta are minute pore-like markings, which appear in the greensand specimens as mere specks of a different nature from the rest of the fossil; but in the gault coprolites they are the distinctly open terminations of fine canals.

(iv.) Contraction-cracks are evident on the surfaces of many specimens, generally filled in with lighter-coloured material. In the coprolites from the Gault the oscula, puncta, canals, cracks, and other cavities are either empty or filled with loose clay; in those from the Greensand all these cavities are infiltrated with phosphatized chalk-marl, containing green grains and sometimes diffused glauconite. Since these infiltrated coprolites of the Greensand are derived from those of the Gault. which are not infiltrated, this filling-in must have taken place after the greensand fossils were washed out of their matrix.

Smooth surfaces of attachment are to be seen on some specimens; and in rare cases the shell on which they grew remains adherent to them.

General Appearances under the Microscope. — Thin sections examined under the microscope vary from colourless to yellowish- brown when transparent, but sometimes they are almost opaque from included earthy matter. Granular patches of a deep red colour are sometimes scattered throughout the fighter-coloured portions. Spicules occur in many sections, presenting some of the most characteristic forms of sponge-spicules, as for example, hexaradiate, triradiate, amate, sinuate, and connecting forms. These spicules are frequently grouped together in a manner which would seem to indicate that they cannot have been washed in from the sea-bed during fossilization. Globular bodies 1/400" in diameter are numerous; they seem to be gemmules. Polycystina and Xanthidia occur in some sections. With polarized light the sections appear distinctly cryptocrystalline, presenting an appearance very nearly resembling that of chalk flints when examined in the same way. A very curious phenomenon may be alluded to here. A number of small circles may be seen in some sections, each of which is marked by a black cross, the arms of which radiate from the centre to the circumference. On turning the analyzer the cross revolves and, when he analyzer has been turned round 90°, is replaced by a complementarity illuminated cross. The explanation of these appearances seems to be as follows: small Globigerina-shells and other similar spaces occur in the coprolite, into which the crystalline apatite which was diffused throughout the fossil has penetrated and crystallized inwards