[Illustration: Fig. 54.--_Sphenopteris latior_, Dawson. Coal-formation, _a_, Pinnule magnified, with traces of fructification.] [Illustration: Fig. 55.--Fructification of Palæozoic ferns, _a_, Thecæ of _Archæopteris_ (Erian). _b_, Theca of _Senftenbergia_ (Carboniferous). _c_, Thecæ of _Asterotheca_ (Carboniferous).] [Illustration: Fig. 56.--Tree-ferns of the Carboniferous. A, _Megaphyton magnificum_, Dawson, restored. B, Leaf-scar of the same, two thirds natural size. B^1, Row of leaf-scars, reduced. C, _Palæopteris Harttii_, scars half natural size. D, _Acadica_, scars half natural size.] Some of these ferns have the more complex kind of spore-case, with a jointed, elastic ring. It is to be observed, however, that those forms which have a simple spore-case, either netted or membranous, and without annulus, are most common in the Devonian and lowest Carboniferous. Some of the forms in these old rocks are somewhat difficult to place in the system. Of these, the species of _Archæopteris_, of the Upper and Middle Erian, are eminent as examples. This type, however, scarcely extends as high as the coal-formation.[CE] Some of the tree-ferns of the Carboniferous present very remarkable features. One of these, of the genus _Megaphyton_, seems to have two rows of great leaves, one at each side of the stem, which was probably sustained by large bundles of aërial roots (Fig. 56). [CE] The pretty little ferns of the genus _Botrychium_ (moonwort), so common in American and European woods, seem to be their nearest modern allies. In the Carboniferous, as in the Erian, there are leaves which have been referred to ferns, but are subject to doubt, as possibly belonging to broad-leaved taxine trees allied to the gingko-tree of China. One of these, represented in Fig. 57, has been found in the coal-formation of Nova Scotia, and referred to the doubtful genus _Noeggerathia_. Fontaine has proposed for similar leaves found in Virginia the new generic name _Saportea_. [Illustration: Fig. 57.--_Noeggerathia disbar_ (half natural size).] Ferns, as might be inferred from their great age, are at the present time dispersed over the whole world; but their headquarters, and the regions to which tree-ferns are confined, are the more moist climates of the tropics and of the southern hemisphere. The coal-swamps of the northern hemisphere seem to have excelled even these favoured regions of the present world as a paradise for ferns.