Among the palæozoic members of the group are other genera closely allied to, but differing from _Calamites_ in some particulars. One of these is _Archæocalamites_, which has a cone almost identical with that of the living Equisetums, as it has no sterile bracts mingled with the umbrella-like sporophylls. Other genera are more complex than those described for _Calamites_, and even in the simple coned _Archæocalamites_ itself the leaves are finely branched and divided instead of being simple scales. But no genus is so completely known as is _Calamites_, which will itself suffice as an illustration of the palæozoic Equisetaceæ. Though the genus, as was pointed out above, shows several important characters differing from those of Equisetum, and parallel to some extent to those of the palæozoic Lycopods, yet these features are more of a physiological nature than a systematic one, and they throw no light on the origin of the family or on its connection with the other Pteridophytes. It is in the extinct family dealt with in the next chapter that we find what some consider as a clue to the solution of these problems. [Illustration: Fig. 111.—Tetrads of Spores of _Calamites_ S, Normal-sized spores; _a_, _b_, &c., aborting spores.] CHAPTER XVI PAST HISTORIES OF PLANT FAMILIES IX. Sphenophyllales The group to which _Sphenophyllum_ belongs is of considerable interest and importance, and is, further, one of those extinct families whose very existence would never have been suspected had it not been discovered by fossil botanists. Not only is the family as a whole extinct, it also shows features in its anatomy which are not to be paralleled among living stems. _Sphenophyllum_ became extinct in the Palæozoic period, but its interest is very real and living to-day, and in the peculiar features of its structure we see the first clue that suggests a common ancestor for the still living groups of Lycopods and Equisetaceæ, which now stand so isolated and far apart. Before, however, we can consider the affinities of the group, we must describe the structure of a typical plant belonging to it. The genus _Sphenophyllum_ includes several species (for which there are no common English names, as they are only known to science) whose differences are of less importance than their points of similarity, so that one species only, _S. plurifoliatum_, will be described.