But the great feature of the time was its dicotyledonous forests, and I have only to enumerate the genera supposed to be represented in order to show the richness of the time in plants of this type. It may be necessary to explain here that the generic names used are mostly based on leaves, and consequently cannot be held as being absolutely certain, since we know that at present one genus may have considerable variety in its leaves, and, on the other hand, that plants of different genera may be very much alike in their foliage. There is, however, undoubtedly a likeness in plan or type of structure in leaves of closely allied plants, and, therefore, if judiciously studied, they can be determined with at least approximate certainty.[DM] More especially we can attain to much certainty when the fruits as well as the leaves are found, and when we can obtain specimens of the wood, showing its structure. Such corroboration is not wanting, though unfortunately the leaves of trees are generally found drifted away from the other organs once connected with them. In my own experience, however, I have often found determinations of the leaves of trees confirmed by the discovery of their fruits or of the structure of their stems. Thus, in the rich cretaceous plant-beds of the Dunvegan series we have beech-nuts associated in the same beds with leaves referred to Fagus. In the Laramie beds I determined many years ago nuts of the Trapa or water-chestnut, and subsequently Lesquereux found, in beds in the United States, leaves which he referred to the same genus. Later, I found in collections made on the Red Deer River of Canada my fruits and Lesquereux’s leaves on the same slab. The presence of trees of the genera Carya and Juglans in the same formation was inferred from their leaves, and specimens have since been obtained of silicified wood, with the microscopic structure of the modern butternut. Still we are willing to admit that determinations from leaves alone are liable to doubt.
[DM] Great allowance has to be made for the variability of leaves of the same species. The modern hazel (C. rostrata) is a case in point. Its leaves, from different parts of the same plant, are so dissimilar in form and size that they might readily be regarded as of different species.
In the matter of names of fossil leaves, I sympathise very strongly with Dr. Nathorst, of Stockholm, in his objection to the use of modern generic names for mere leaves, and would be quite content to adopt some non-committal termination, as that of “phyllum” or “ites” suggested by him. I feel, however, that almost as much is taken for granted if a plant is called Corylophyllum or Corylites, as if called Corylus. In either case a judgment is expressed as to its affinities, which if wrong under the one term is wrong under the other; and after so much has been done by so many eminent botanists, it seems inexpedient to change the whole nomenclature for so small and questionable an advantage. I wish it, however, to be distinctly understood that plants catalogued on the evidence of leaves alone are for the most part referred to certain genera on grounds necessarily imperfect, and their names are therefore subject to correction, as new facts may be obtained.
The more noteworthy modern genera included in the Dakota flora, as catalogued by Lesquereux, are the following: Liquidambar, the sweet-gum, is represented both in America and Europe, the leaves resembling those of the modern species, but with entire edges, which seems to be a common peculiarity of Cretaceous foliage.[DN] Populus (poplar), as already stated, appears very early in Greenland, and continues with increasing number of species throughout the Cretaceous and Tertiary. Salix (willow) appears only a little later and continues. Of the family Cupuliferæ we have Fagus (beech), Quercus (oak), and Castanea (chestnut), which appear together in the Dakota group and its equivalents. Fruits of some of the species are known, and also wood showing structure. Betula (birch) is represented by a few species, and specimens of its peculiar bark are also common. Alnus (alder) appears in one species at least. The genus Plat anus ([Fig. 71]), that of the plane-trees, represented at present by one European and one American species, has several species in the Cretaceous, though the plane-trees seem to culminate in the early part of the succeeding Eocene, where there are several species with immense leaves. The large leaves, known as Credneria, found in the Cenomanian of Europe, and those called Protophyllum ([Fig. 72]) in America, appear to be nearer to the plane-trees than to any others, though representing an extinct type. The laurels are represented in this age, and the American genus Sassafras, which has now only one species, has not one merely but several species in the Cretaceous. Diospyros, the persimmon-tree, was also a Cretaceous genus.
[DN] With reference to this, something may be learned from the leaves of modern trees. In these, young shoots have leaves often less toothed and serrated than those of the adult tree. A remarkable instance is the Populus grandidentatus of America, the young shoots of which have entire leaves, quite unlike except in venation those of the parent tree, and having an aspect very similar to that of the Cretaceous poplars.
Fig. 71.—Platanus nobilis, Newberry, variety basilobata. Laramie. Much reduced.
