the Cycadophyte branch is not distinguishable until the Mesozoic.
Historically, therefore, the Cordaitales must be considered as the second group of Gymnosperms. Their connection with an ancient fern stock is evident in their structure, but they have lost many of the fern characters that were retained by Cycadofilicales. The fact that Cordaitales are much further from ferns than are the Cycadofilicales is perhaps the best proof that they have come from the ferns by way of the Cycadofilicales. The combination of changes involved in their structure is all in the direction of the later Conifers, as, for example, the branching stem (constituting what is called "the habit") with its thick cylinder of secondary wood, the narrow and entire leaves, and the cones (strobili). It would not fit the purpose of this presentation to include the changes in the more intimate structures, since their nature and significance can be appreciated only by the special students of the group, but they are just as striking as the more obvious changes mentioned.
The Gymnosperm vegetation of the Paleozoic, therefore, comprised two great genetic groups: the Cycadofilicales, representing the primitive Gymnosperm stock that differentiated from the ferns; and the Cordaitales, representing the primitive Coniferophyte stock that differentiated from Cycadofilicales more ancient than those we know.
In the Mesozoic flora the Gymnosperms were represented by four great groups, evidently derived from the two Paleozoic groups. As stated above, the Cycadophyte branch became distinct, and for a long time all of its representatives were thought to be Cycads. For this reason, the Mesozoic has been called the "age of Cycads," so far as the vegetation is concerned. It is one of the triumphs of American paleobotany that it has put on a firm basis our knowledge of the great Cycadophyte group of the Mesozoic, and has shown that it is quite different from the modern Cycads. The group is called Bennettitales, and although a few forms from foreign localities have been known for a long time, it remained for Mesozoic deposits of the United States and Mexico to reveal a remarkably rich display of forms in admirable preservation. The investigation of this material has been carried on chiefly by Dr. G. E. Wieland of the Yale Museum.
The Bennettitales, therefore, are the so-called "fossil Cycads" of the Mesozoic. So far as the records show, they are restricted to the Mesozoic, so that they represent an extinct Mesozoic group, just as there are two extinct Paleozoic groups. Of course it is not only conceivable, but also probable that the Paleozoic Cycadofilicales, from which Bennettitales were derived, continued into early Mesozoic; and that the Mesozoic Bennettitales began to differentiate in late Paleozoic. The external appearance of Bennettitales justifies their early assignment to the Cycads, for the whole habit is Cycadean. The stems are either tuberous or cylindrical, and crowned by a rosette of large, fern-like
