616 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
cases they have become isolated in the sporangial cavity (fig. 72). They measure 
about 50 n in greatest diameter. Spores are also found distributed through the 
peaty matrix in which the plant is embedded. 
Diagnosis. 
Hornea, Kidston and Lang, n.g. 
Plant rootless and leafless. Stems arising from protocorm-like rhizomes, 
dichotomously branched. Sporangia terminal on ultimate branches, with a sterile 
columella projecting from the base into the sporangial cavity, and cuticularised 
spores developed in tetrads. 
Hornea Lignieri, Kidston and Lang, n.sp. 
Plant small, consisting of a lobed rhizome from which arise stems which branch 
dichotomously and range from 2 mm. in diameter downwards. Stele of stem with 
a zone of phloem surrounding the xylem composed of small central and wider peri- 
pheral tracheides. Sporangia cylindrical, terminal on branches, indehiscent, with 
thick wall composed of thickened epidermis, thin-walled tissue, and persistent tapetal 
layer. Sterile columella composed of thin-walled elongated cells extending from base 
to near top of sporangium. Homosporous. Spores about 50 p in diameter. 
Locality. — Muir of Rhynie, Aberdeenshire. 
Horizon. — Old Red Sandstone (not younger than the Middle Division of the Old 
Red Sandstone of Scotland). 
Classification of Rhynia and Hornea. 
Rhynia and Hornea, while distinguished generically, agree so closely in the 
simplicity of their organisation that they must be regarded as genera of the same 
Family. For this Family we suggest the name Rhyniacese. 
It is characterised by the plants being rootless and leafless and composed of 
rhizomes which bear rhizoids, branched aerial stems, and terminal sporangia. The 
vascular system is correspondingly simple, the central stele having a cylindrical 
strand of xylem either composed of similar tracheides or with a distinction of central 
and peripheral xylem. 
The Family Rhyniacese comes into the class of Vascular Cryptogams to which 
we have given the name of Psilophytales (Part I, p. 780), since “the sporangia are 
borne at the ends of branches of the stem without any relation to leaves or leaf- 
like organs.” 
The consideration of more complicated types of the Psilophytales can be deferred 
until Asteroxylon is described. 
