Differences in age as well as different degrees of decortication have contributed in no small measure to the institution of generic and specific names which more recently acquired knowledge has shown to be superfluous. _a._ _Knorria._ The designation Knorria, after a certain G. W. Knorr of Nürnberg, was proposed by Sternberg in 1826[273] for casts of Palaeozoic stems of a type figured more than a century earlier by Volkmann[274]. Goeppert, in his earlier works, published drawings of fossil stems which he referred to Sternberg’s genus: one species he at first called _Didymophyllum Schollini_. He afterwards[275] described some specimens which showed that the features characteristic of _Knorria_ may occur on partially decorticated stems with leaf-cushions of the true Lepidodendron type. His specimens, preserved in the Breslau Museum, demonstrate the accuracy of his drawings and conclusions. Goeppert, and after him Balfour[276], drew attention to the different appearances presented by branches of _Araucaria imbricata_ when preserved with the surface intact and after partial decortication, as illustrating possible sources of error in the determination of fossil stems. Although it is now a well-established fact that fossils bearing the name _Knorria_ are imperfect lepidodendroid stems, the use of the term may be conveniently retained for descriptive purposes. The specimen from the Commentry coal-field of France, shown in fig. 156, affords some excuse for the institution of several generic names for different states of preservation or decortication of one species. The cortical level exposed at _e_ is characterised by spirally disposed peg-like ridges with truncated apices: it is this form of cast which is usually designated _Knorria_. The ridges vary in size and shape in different types of stem; they may be narrow as shown at _e_, fig. 156, or short and broad with rounded distal ends. In some cases they are forked at the apex, as in the partially decorticated specimen of _Lepidodendron Veltheimianum_ represented in fig. 185, A. [Illustration: FIG. 156. A dichotomously branched Lepidodendroid stem (_Knorria mirabilis_ Ren. and Zeill.). (After Renault and Zeiller.) (¼ nat. size.) The original specimen is in the Natural History Museum, Paris. _a–g_, surface features exposed as the result of different degrees of decortication. (See vol. I. p. 102, fig. 23).]