It has been stated, on evidence which is unsatisfactory, that _Sphenophyllum_ possesses two kinds of spores. While regarding the genus as homosporous on the evidence before us, it is interesting to find that cases occur in which the spores in the same sporangium exhibit a marked difference in size. Attention has been called by Williamson and Scott[13] to variation in the dimensions of spores: a more pronounced difference in size has been recorded by Mr Thoday[14] who gives 120μ as the maximum and 90μ as the minimum diameter of the spores in a cone of _Sphenophyllostachys Dawsoni_. The presence of several abortive spores in the sporangium (fig. 116) containing the larger spores favours the view that this difference in size may be the first step towards the development of heterospory. It is clear that the types of strobilus designated _Sphenophyllostachys_ (figs. 112–114) present a divergence of characters too great to be comprised under one genus; but in the absence of fuller information, we cannot do otherwise than follow the only logical custom of grouping them together as examples of strobili borne by plants which, in the present state of our knowledge, are most conveniently referred to the genus _Sphenophyllum_. _Cheirostrobus._ This generic name was applied by Dr Scott[15] to a calcified cone obtained by Mr James Bennie in 1883 from the Lower Carboniferous plant-beds of Pettycur near Burntisland on the Firth of Forth. _Cheirostrobus_ is distinguished from _Sphenophyllostachys_ by its greater breadth (3.5 cm.); externally it agrees more closely with the fertile shoots of _Lepidodendron_ than with those of _Sphenophyllum_. A single vascular cylinder having the form of a fluted Doric column (fig. 117, B, _x_) occupies the axis of the cone: it consists for the most part of reticulate tracheae which tend to assume a short or isodiametric form in the central region; the smaller protoxylem tracheids with the spiral form of pitting constitute the sharp and prominent ridges at the periphery of the xylem-cylinder. In the outer part of the cylinder the metaxylem[16] consists exclusively of tracheae, but towards the centre of the axis these are associated with numerous parenchymatous cells.