of the best, it is a serious mistake to make use of recent generic designations. If we consider the difficulties which would attend an attempt to determine the leaves, fragments of stems and other detached portions of various recent genera, we can better appreciate the greater probability of error in the case of imperfectly preserved fossil fragments. [Sidenote: EXTERNAL RESEMBLANCE.] The portions of stems represented in figures 20 and 21, exhibit a fairly close resemblance to one another; in the absence of microscopical sections or of the reproductive organs it would be practically impossible to discriminate with any certainty between fossil specimens of the plants shown in the drawings. Examples such as these, and many others which might be cited, serve to illustrate the possibility of confusion not merely between different genera of the same family, but even between members of different classes or groups. The long slender branches of the _Polygonum_ represented in (fig. 21) would naturally be referred to _Equisetum_ in the absence of the flowers (fig. 20 _B_), or without a careful examination of the insignificant scaly leaves borne at the nodes. The resemblance between _Casuarina_ and _Ephedra_ and the British species of _Equisetum_, or such a tropical form as _E. debile_, speaks for itself. [Illustration: FIG. 20. _A._ _Restio tetraphylla_ Labill. (Monocotyledon). _B._ _Equisetum variegatum_ Schleich. ╮ (Vascular Cryptogam). _C._ _Equisetum debile_ Roxb. ╯ _D._ _Casuarina stricta_ Dryand. (Dicotyledon). _E._ _Ephedra distachya_ Linn. (Gymnosperm). (_A_–_E_ ½ nat. size).] [Illustration: FIG. 21. _Polygonum Equisetiforme_ Sibth. and Sm. _A._ Showing habit of plant. ½ nat. size. The two flowers towards the apex of one branch, drawn to a larger scale in _B_. _C._ Node with small leaf and ochrea characteristic of _Polygonaceæ_. From a plant in the Cambridge Botanic Garden.] [Illustration: FIG. 22. _Kaulfussia æsculifolia_ Blume. From a specimen from Java in the British Museum herbarium. ⅓ nat. size.] Endless examples might be quoted illustrating the absolute futility, in many cases, of relying on external features even for the purpose of class distinction. An acquaintance with the general habit and appearance of only the better known members of a family, frequently leads to serious mistakes. The specimen shown in fig. 22 is a leaf of a tropical fern _Kaulfussia_, a genus now living in South-eastern Asia, and a member of one of the most important and interesting families of the Filicinæ, the Marrattiaceæ; its form is widely different from that which one is accustomed to associate with fern fronds. It is unlikely that the impression of a sterile leaf of _Kaulfussia_ would be recognised as a portion of a fern plant.