The occurrence of small ramifying tubes in recent and fossil corals, fish-scales, and bones was long ago pointed out by Quekett[195], Kölliker[196], Rose[197] and other writers[198]. These narrow tubular cavities have generally been attributed to the boring action of some parasitic organism, either a fungus or an alga. In 1876 Duncan published two important papers[199] dealing with the occurrence of such tubes in recent corals, as well as in the calcareous skeleton of _Calceolina_, _Goniophyllum_ and other Palaeozoic, Mesozoic and Tertiary species of corals. This writer attributed the formation of the cavities in the case of the fossil species to the action of a fungus which he named _Palaeachlya perforans_, and considered as very nearly related to _Achlya penetrans_ found in the “dense sclerenchyma” of recent corals. In fig. 27 A. is reproduced one of the drawings given by Rose[200] in his paper published in 1855; it shows a section of a fish-scale from the Kimeridge clay which has been attacked by a boring organism. Rose attributes the dichotomously branched canals to some “infusorial parasite.” [Illustration: FIG. 27. A, Section of a fish-scale from the Kimeridge Clay, showing branched canals, made by a boring organism, × 85. B, Section of a Solen shell, penetrated in all directions by the boring thallus of _Ostracoblabe_ (a fungus?), × 330. C, Piece of the thallus of _Ostracoblabe_ isolated by decalcification, × 745. A, after Rose. B and C, after Bornet and Flahault.] In the important paper by MM. Bornet and Flahault on perforating algae a full description is given of various boring forms belonging to the Chlorophyceae and the Cyanophyceae[201]. The canals which these algae produce in calcareous shells and other hard substances are of the same type as those previously described in fossil corals, fish-scales and bones. In dealing with living perforating Thallophytes the colour and other cell-contents often enable us to distinguish between algae and fungi, but in fossil specimens such tests cannot be applied. The fossil tubular borings may or may not show traces of the transverse septa and reproductive cells; it is often the case that no trace of the organism has been left, but only the canals by which it penetrated the calcareous or bony skeleton. In some of the examples of _Palaeachlya_ figured by Duncan there appear to be numerous spores in some of the sections, but it is generally a very difficult and often an impossible task to discriminate between the borings of fungi and algae in fossil material.