~~ 457 — 
just a little above each leaf scar which is often more distinctly hexa- 
gonal, or more broadly ovate-hexagonal. The ribs often exhibit va- 
rious types of topographical ornamentation, such as rugosity, shagree- 
ning, fretting, banding, etc., which take part in their specific characte- 
rizaction. 'requently the leaf scars in this section are situated on more 
or less distinct and imperfectly hexagonal leaf cushions growing 
out of the ribs. In the large trunks of some of the species in the 
Rhytidolepis section the ribs sometimes attain a width ofover three 
centimeters. The stems of this type are among the most easily re- 
cognized of the Palaeozoic relatives of tha Lycopods. 
In the Sub-Sigilliariae there are t\o types which are often indis- 
tinguishable from each other: Clathraria, in which the hexagonal 
leaf-cushions are close and spirally arranged; and Leiordermaria, in 
which the leaf scars are more or less distantly arranged on the essen- 
tially flat cortex. In some species both Clathrarian and Leiodermarian 
forms are seen on the same stem. In the forms with flat cortex, having 
an inconspicus if any development of a leaf cushion, the surface is 
often lightly corrugated or otherwise ornamented in some particular 
design which serves as a diagnostic character. The leaf-scars of the 
Sub-Sigiliariae are hexagonal, with the greater diameter transversely 
between the two usually acute lateral angles. 
The species from Brasil belong to the Swb-Sigilliariae, Evidence 
of branching is rare in Sigillaria, though the trees were often of 
very large size, specimens being found to measure as much as six 
feet in diameter, or nearly 100 feet in length. In all the types of Si- 
gillaria the leaf scars are spirally disposed. Just above each of the scars 
there is aslight depression or punctiform pit marking the position of 
the ligular cavity. Wherever individual leaf cushions are to some degree 
in evidence they show more or less distinctly a hexagonal form, and 
have the leaf scar, which is usually slightly oblique to the plane of 
the trunk, located nearer the upper side of the cushion. When any 
of the angles of the leaf-scar become rounded, the rounding first 
appears in the two angles on the upper side. The more distinctly 
hexagonal form of scar is seen in the Sub-Sigillariae. 
The leaf scar contains three small cicatricules at or above the 
middle, as in Lepidodendron. The central one, or nerve trace, is 
usually punctiform or broadly V shaped; the two lateral (Pari- 
chncs) are generally elongated more or less distinctly in the longi- 
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