[Illustration: Fig. 21.--_Lepidodendron_ and _Psilophyton_ (Erian, New Brunswick). A, _Lepidodendron Gaspianum_. B, C, _Psilophyton elegans_.]
_Arthrostigma_, which is found in the same beds with Psilophyton, was a plant of more robust growth, with better-developed, narrow, and pointed leaves, borne in a verticillate or spiral manner, and bearing at the ends of its branches spikes of naked sporocarps, apparently similar to those of _Psilophyton_ but more rounded in form. The two genera must have been nearly related, and the slender branchlets of _Arthrostigma_ are, unless well preserved, scarcely distinguishable from the stems of _Psilophyton_.[AU]
[AU] Reports of the author on "Devonian Plants," "Geological Survey of Canada," which see for details as to Erian Flora of northeastern America.
If, now, we compare the vegetation of these and similar ancient plants with that of modern Rhizocarps, we shall find that the latter still present, though in a depauperated and diminished form, some of the characteristics of their predecessors. Some, like _Pilularia_, have simple linear leaves; others, like _Marsilea_, have leaves in verticils and cuneate in form; while others, like Azolla and Salvinia, have frondose leaves, more or less pinnate in their arrangement. The first type presents little that is characteristic, but there are in the Erian sandstones and shales great quantities of filamentous and linear objects which it has been impossible to refer to any genus, and which might have belonged to plants of the type of _Pilularia_. It is quite possible, also, that such plants as _Psilophyton glabrum_ and _Cordaites angustifolia_, of which the fructification is quite unknown, may have been allied to Rhizocarps. With regard to the verticillate type, we are at once reminded of _Sphenophyllum_ (Fig. 20), which many palæobotanists have referred to the _Marsiliacæ_, though, like other Palæozoic Acrogens, it presents complexities not seen in its modern representatives. _S. primævum_ of Lesquereux is found in the Hudson River group, and my _S. antiquum_ in the Middle Erian. Besides these, there are in the Silurian and Erian beds plants with verticillate leaves which have been placed with the Annulariæ, but which may have differed from them in fructification. _Annularia laxa_, of the Erian, and _Protannularia Harknessii_, of the Siluro-Cambrian, may be given as examples, and must have been aquatic plants, probably allied to Rhizocarps. It is deserving of notice, also, that the two best-known species of _Psilophyton_ (_P. princeps_ and _P. robustius_), while allied to Lycopods by the structure of the stem and such rudimentary foliage as they possess, are also allied, by the form of their fructification, to the Rhizocarps, and not to ferns, as some palæobotanists have incorrectly supposed. A similar remark applies to _Arthrostigma_; and the beautiful pinnately leaved _Ptilophyton_ may be taken to represent that type of foliage as seen in modern Rhizocarps, while the allied forms of the Carboniferous which Lesquereux has named _Trochophyllum_, seem to have had sporocarps attached to the stem in the manner of _Azolla_.