o. 
arranged “steles. with “star rings” inthe central paren- 
chymatous region, and in some forms successive cambis 
in the cortical zone. hese forms are far more complex 
than JZ. Olseniac, and, indeed, represent some of the most 
bizarre modifications of vascular tissue among Paleozoic 
plants. 
Comparison of the new species must therefore be made 
with forms described from the Kuropean and American 
Carboniferous. In many respects WM. Olseniae is within 
the range of variation of the Carboniferous forms, espe- 
cliully in anatomical details, such as tracheary pitting, 
size of the stelar units, endocentricity of the secondary 
evlinders, and structure of the rays. Differences in the 
structural variation of these anatomical features comprise 
the primary basis for establishment of several species of 
American forms, although these differences may actually 
be merely structural variables of the same species. In 
the absence of more complete material from a single 
plant, however, the establishment of species categories 
based on anatomical differences in fragmentary specimens 
seems the only reasonable procedure in the taxonomic 
treatment of Paleozoic plants and other extinct groups 
of plants. On the basis of the structure of the primary 
xylem and of the secondary xylem, J. Olseniae differs in 
no fundamental way from the carboniferous forms. If 
the number of component “‘steles’” in the ‘‘polystele™ 
is considered a basic taxonomic character in Wedullosa 
(which may reasonably be questioned in certain cases), 
M. Olseniae differs from previously described forms in 
that it consistently possesses three to five “‘steles.”* Car- 
boniferous forms described in the literature may be 
vrouped into two major categories: 1) those with two 
to four “*steles’”: and 2) those with six to eight ‘‘steles.”* 
In the first group those with two ‘‘steles’” are M. dis- 
telica Schopt (1989) and MW. pandurata Stewart (1951): 
[ 198 | 
