252 de Fr aine. — On Medullas a centra fills, a New Species of 
8, and between sections 9 and 24, could be estimated, so that the rapidity 
with which the changes occurred in the series of sections could not be 
determined. 
IL General Structure. 
The specimen consists of a portion of a stem surrounded by adherent 
leaf-bases. The diameters of the stem in the topmost section are roughly 
5 cm. x 1*5 cm., including the leaf-bases ; this estimate of the longer diameter 
is certainly too low, however, for part of one of the leaf-bases has either been 
torn off or else destroyed at some time during the process of petrifaction. 
The shape of the stem with its covering of leaf-bases was thus distinctly 
flattened (PI. I, Fig. i), though the form is, in all probability, exaggerate- , 
by lateral pressure. 
Text-fig. i. Diagram of slide b. This diagram is intended to serve as a key to Plate XV, 
Fig. 1. x 2. 
The ends of the long axis of the stem are occupied by prominent 
masses of tissue which represent leaf-bases, still in continuity with the stem. 
Practically the whole of one of these is present (lb. 2 , Text-fig. 1), though its 
tissues are much displaced by the intrusion of Stigmarian rootlets ; two bands 
of sclerenchymatous fibres, sc . 1 and sc. 2 , mark the limit between the tissues 
of the adherent leaf-base and those of the stem proper. 
Nearly opposite to this leaf-base, at the other end of the long diameter 
of the stem, is a second leaf-base, lb. 1 , much less well preserved than lb 2 ; 
its outer parts have been almost entirely removed. A broad zone of 
anastomosing groups of sclerenchymatous fibres serves to delimit the tissues 
of this leaf-base ; this zone, sc.b., is very closely similar in its construction 
to the hypodermal layer of the leaf-base; it differs in the much more compact 
