Page:Quarterly Journal of the Geological Society of London, vol. 32.djvu/283

Rh generally speaking, were filled either with sand and minute fossils of the same period as the mass, or with crystalline carbonate of lime. The specimens were very perfect and had not been rolled.

In investigating these parasitic growths and perforations in recent and fossil specimens, it is necessary to use thin sections for transmitted light; but a thick section often exhibits the refractive tubes when reflected light is employed. The sections of the fossils should be carefully made, and scratches on their surfaces noted; and in order to prevent erroneous interpretations, it is as well not to record any tubes as parasitical in their formation unless they contain matters resembling more or less those of the recent forms, and unless they can be traced in and amongst the normal tissue, and not only on the surface.

The best plan is to examine, first of all, that part of the section of the coral or shell which was formerly exposed to the sea; there it is exceptional not to find one or more straight dark lines passing from close to the external margin or old surface inwards at different angles (fig. 6). (A magnifying power of 350 linear is necessary, and careful and good illumination and definition.) When they are satisfactorily seen it is necessary to examine them throughout their length, and to establish, if possible, their relation with others and with the outside, and to notice their contents.

These perforations or tubes, and the concavities and little loculi with which they are often connected at the surface, cannot be mis- taken for Cliona-borings; for these last are larger, contain spicules, and do not present the long and often tubular branchings of the vegetable parasites, which, moreover, never contain spicula. But the loculi, when some of the Algae get into the corals, do often resemble the results of the early efforts of Cliona to perforate; and it is quite possible that the Alga3 may have subsequently occupied the space where a Cliona had been at work ineffectually.

Their length, minute size, and general characters separate them from some very ill-defined organic perforations seen in Belemnites and modern shells, and which Fischer and Quenstedt have termed Dendrinæ. It is hardly necessary to suggest that the edges of the planes of crystallization in no way resemble the tubes.

Appearance of the tubes and other parasitic productions in Goniophyllum pyramidale (Plate XVI.).—The microscopical elements to be observed, are:—

1. Tubes which have no proper wall, and which are excavations out of the coral-structures. They are found (α) just beneath the surface, running parallel to it; but these are rare (fig. 2); (β) running more or less inwards at different angles to the surface, many being found Dear to the edge of the coral- wall, and a few far away towards the interior (figs. 6 & 9). These last-mentioned tubes do not vary much in size, and average in diameter about 0⋅008 in. Their calibre does not alter in different parts of their course, which is rarely curved, usually straight, and occasionally branching, the branches being often as large as the parent tube.