The characters of the Cycads as they are now living prove them to be an extremely primitive group, and therefore presumably well represented among the fossils; and indeed among the Mesozoic rocks there is no lack of impressions which have been described as the leaves of Cycads. There is, however, very little reliable material, and practically none which shows good microscopic structure. Leaf impressions alone are most unsafe—more unsafe in this group, perhaps, than in any other—for reasons that will be apparent later on, and the conclusions that used to be drawn about the vast number of Cycads which inhabited the globe in the early Mesozoic must be looked on with caution, resulting from the experience of recent discoveries proving many of these leaves to belong to a different family. There remain, however, many authentic specimens which show that _Cycas_ certainly goes back very far in history, and specimens of this genus are known from the older Mesozoic rocks. We cannot say, however, as securely as used to be said, that the Mesozoic was the “Age of Cycads”, although it was doubtless the age of plants which had much of the external appearance of Cycads. From the Palæozoic we have no reliable evidence of the existence of Cycads, though the plants of that time included a group which has an undoubted connection with them. Indeed, so far as fossil evidence goes, we must suppose that the Cycads, since their appearance, possibly at the close of the Palæozoic, have never been a dominant or very extensive family, though they grew in the past all over the world, and in Europe seem to have remained till the middle of the Tertiary epoch. CHAPTER XII PAST HISTORIES OF PLANT FAMILIES V. Pteridosperms This group consists entirely of plants which are extinct, and which were in the height of their development in the Coal Measure period. As a group they are the most recently discovered in the plant world, and but a few years ago the name “Pteridosperm” was unknown. They form, however, both one of the most interesting of plant families and one of the most numerous of those which flourished in the Carboniferous period. To mention first the vital point of interest in their structure, they show _leaves which in all respects appear like ordinary foliage leaves, and yet bear seeds_. These leaves, which we now know bore the seeds, had long been considered as typical fern leaves, and had been named and described as fern leaves. There are two extremely important results from the discovery of this fossil group, viz. that leaves, to all appearance like ordinary foliage, can directly bear seeds, and that the leaves, though like fern leaves, bore seeds like those of a Cycad. As the name _Pteridosperm_ indicates, the group is a link between the ferns and the seed-bearing plants, and as such is of special interest and value to botanists.