690 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
brief recapitulation, with the addition of one or two details, is needed here. The 
question of the presence of a distinct pericycle and endodermis will be postponed 
until the smaller rootlets have also been taken into consideration. 
Tn all cases where the cortex is preserved, it consists of three well-marked zones : 
(1) an inner cortex, of continuous parenchyma, immediately surrounding the central 
cylinder ; (2) a middle cortex, which is /acunar, consisting of radial plates of tissue, 
separated from one another by wide intercellular spaces ; (3) an outer cortex, con- 
sisting, like the inner zone, of continuous parenchyma.* The relative thickness of 
the three zones varies considerably in different specimens, as is sufficiently shown in 
the figures cited. The lacunz, however, are always arranged in a single circle only. 
The large root shown in photograph 3 has a somewhat peculiar cortex, which, at the 
first glance, appears to consist entirely of solid parenchyma. More careful examina- 
tion, however, reveals the existence of a rather narrow lacunar zone (see Plate 17, 
fig. 11, 1z.) in the usual intermediate position. The trabecule have been crushed 
in upon the inner cortex, so as almost to obliterate the lacunz between them. 
The specimen shown, in transverse section, in photograph 4, has a cortex of the 
“ Astromyelon” type, but ditfering considerably from the more usual form. It is, in 
all probability, specifically distinct from the more ordinary examples. The trabeculee 
are much more numerous than usual, and the lacunee between them are narrower in a 
corresponding degree. At some places the lacun@ are not empty, but contain large, 
irregular, rounded cells. Similar intralacunar cells have been found occasionally in 
other specimens ; their nature will be discussed below (p. 693). 
The structure of the trabeculz is well shown in longitudinal or oblique sections, 
from which it is evident that they formed continuous vertical partitions, each of 
which was only one cell in thickness (see fig. 8; also “ Organization,” Part XIL., 
Plate 31, fig. 4). The cells of which they are composed are cylindrical in shape, 
so that the partitions, as seen in tangential section, have a moniliform appearance. 
The inner cells of each trabecula are usually short, while those further to the exterior 
are often much elongated in the radial direction. Some further details as to the 
cortex will be given when the rootlets are considered. 
The Rootlets. 
Under the name “rootlets” we group those smaller specimens, which, while 
agreeing in other respects with the forms already described, differ from them in 
having little or no pith, and no primary rays, so that the groups of primary xylem 
either meet at the centre of the stele, or are, at least, laterally confluent. Specimens of 
this kind were included by Hick and Casa in their genus Myriophylloides,t the type- 
specimen of which, as we have seen, had a considerable pith and typical Astromyelon 
* See the figures in “ Organization,” Part XII. 
+ Loc cit., ‘Proc. Yorkshire Geol. and Polytechnic Soc.,’ vol. 8, p- 877, 1884. See also Renaotr, 
“Nouvelles Recherches,” p. 101, 1885, 
