MINUTE STRUCTURE. 33 



these differences can be recognised with absolute certainty, provided the internal 

 structure has not been destroyed in the process of fossilisation ; and that both in 

 consistency and in amount they are perfectly adequate for the discrimination of 

 the different species, or for the establishment of generic distinctions. Moreover, 

 when once the peculiarities of the microscopic structure have been fully recog- 

 nised, it is usually quite easy to correlate these with small and otherwise hardly 

 recognisable external characteristics, so that it becomes, in general, a comparatively 

 light matter to determine the position of a given specimen by a mere macroscopic 

 examination. I do not mean to assert that the generic divisions of the Stroma- 

 toporoids are all rigidly marked off by their minute structure, for there are types, of 

 an inosculant character, which it is difficult to place definitely in one genus rather 

 than in another. Nor do I mean to assert that one does not meet with specimens 

 which can only with difficulty and uncertainty be determined even with the help of 

 microscopic sections. I do mean to assert, however, that the minute microscopic 

 structure of the skeleton of the Stromatoporoids may be relied upon in the 

 determination of species or genera, to just the same extent, with just as much 

 certainty, and under precisely the same limitations, as in the case of the Corals or 

 the Polyzoa. 



There is, however, a special and exceptional difficulty in the case of the Stroma- 

 toporoids in the preparation of thin sections, which should not be passed over 

 wholly without remark. As will be seen immediately, the skeleton of the Stroma- 

 toporoids consists essentially of two sets of elements, one radial and the other 

 tangential, as regards the whole specimen, and therefore in the main intersecting 

 each other at right angles. Hence, two sets of sections must in all cases be 

 prepared, viz. one section parallel with the radial (or vertical) elements of the 

 skeleton, and one at right angles to this, parallel with the tangential (or horizontal) 

 elements of the skeleton. If the two component elements of the skeleton were 

 rectilinear, and cut each other accurately at right angles, it would be an easy 

 matter to prepare such sections. As a matter of fact, however, the vertical or 

 radial elements of the skeleton are usually flexuous, and the tangential or concentric 

 elements are invariably more or less curved ; so that it is a matter of more or less 

 difficulty to prepare slides which shall be accurately parallel to either of these sets 

 of elements. It is, however, absolutely necessary to secure approximate parallelism 

 to the constituent elements of the skeleton, if the sections are to yield reliable 

 information. A very slight obliquity — especially in vertical sections — causes a 

 distortion of the structure, which may be recognised and allowed for by the 

 experienced observer, but which is exceedingly likely to mislead anyone who has 

 not examined a large series of specimens. For the same reason, the beautiful 

 polished sections prepared by lapidaries are in many cases of comparatively little 

 value for working purposes, as they are cut at all angles of obliquity to the 



