17H Scientific Proceedings, Royal Dublin Society. 



to fill in the great gap between ferus and the lower forms. The general 

 character of the plant, which I propose to call Forbesia cancellata in honour 

 of Edward Forbes, the discoverer of Archceopteris hibernica, is indicated in 

 PI. XIII, fig. 1. 



The axis is dichotomously branched several times, and gives off at long 

 intervals leaves which show a rachis, itself dichotomously divided. The 

 laminated part of the leaf is also dichotomously divided, and ends in 

 obtuse, simple, or bilobed wedge-shaped segments (fig. 1). Dichotomy of 

 axis and frond is thus a pronounced feature. Though the terms " axis " and 

 " frond " are used, there is really no means of distinguishing one from the otlier 

 structurally. Both axis and frond are honeycombed structures. They consist 

 of numerous air-chambers partitioned off from one another by septa, which 

 appear strengthened by a band of sclerotic tissue. In the specimens some of 

 the air-chambers are free from mineral deposit, and under magnification 



Fig. 1. Fie. 2. 



sliow a surface marked by fine longitudinal lines with connecting cross-lines. 

 In each of the microscopic spaces so formed a dot-like depression is observable 

 (fig. 2). In other words, the surface of the air-chamber is lined by parenchy- 

 matous cells whose cavities are represented by the minute central pits. By 

 magnification what appears to be a band of sclerotic tissue can also be seen 

 running through the septum between the adjoining air-chambers. Just 

 beneath the surface of the axis, on either side, there is observable in some 

 cases (PI. XIV, fig. 5) a pronounced vertical strand which is connected with 

 the more or less rectangular reticulum formed in the septa of the air 

 chambers in the axis. This strand curves out into the rachis of the frond. 

 It appears to be a strengthening strand. The frond shows, as mentioned, 

 exactly the same structure as the axis. It differs from it in position (being 

 a lateral outgrowth from it), and in the lamination or flattening out of 



