There is a common source of danger in attempting to carry too far the venation characters as tests of affinity. The parallel venation of Monocotyledons is by no means a safe guide to follow in all cases as a distinguishing feature of this class of plants. In addition to such leaves as those of the Gymnosperm _Cordaites_ and detached pinnæ of Cycads, there are certain species of Dicotyledons which correspond in the character of their venation to Monocotyledonous leaves. _Eryngium montanum_ Coult., _E. Lassauxi_ Dcne., and other species of this genus of _Umbelliferæ_ agree closely with such a plant as _Pandanus_ or other Monocotyledons; similarly the long linear leaves of _Richea dracophylla_, R. Br., one of the Ericaceæ, are identical in form with many monocotyledonous leaves. Instances might also be quoted of monocotyledonous leaves, such as species of _Smilax_ and others which Lindley included in his family of Dictyogens which correspond closely with some types of Dicotyledons[142]. Venation characters must be used with care even in determining classes or groups, and with still greater reserve if relied on as family or generic tests. It is too frequently the case that while we are conversant with the most detailed histological structure of a fossil plant stem, its external form is a matter of conjecture. The conditions which have favoured the petrifaction of plant tissues have as a rule not been favourable for the preservation of good casts or impressions of the external features; and, on the other hand, in the best impressions of fern fronds or other plants, in which the finest veins are clearly marked, there is no trace of internal structure. It is, however, frequently the case that a knowledge of the internal structure of a particular plant enables us to interpret certain features in a structureless cast which could not be understood without the help of histological facts. A particularly interesting example of anatomical knowledge affording a key to apparently abnormal peculiarities in a specimen preserved by incrustation, is afforded by the fructification of the genus _Sphenophyllum_. Some few years ago Williamson described in detail the structure of a fossil strobilus (_i.e._ cone) from the Coal-Measures, but owing to the isolated occurrence of the specimens he was unable to determine the plant to which the strobilus belonged. On re-examining some strobili of _Sphenophyllum_, preserved by incrustation, in the light of Williamson’s descriptions, Zeiller was able to explain certain features in his specimens which had hitherto been a puzzle, and he demonstrated that Williamson’s cone was that of a _Sphenophyllum_. Similar examples might be quoted, but enough has been said to emphasize the importance of dealing as far as possible with both petrifactions and incrustations. The facts derived from a study of a plant in one form of preservation may enable us to interpret or to amplify the data afforded by specimens preserved in another form.