Lycoftodiales. 159 
genus, and should further investigation reveal other homosporous 
cones, and no megasporangia, a fair presumption of homospory in 
Spencerites would be established. A homosporous condition would 
agree well with the relative primitiveness sometimes attributed to 
Spencerites. 
Whatever the relationship between Spencerites, Lepidostrobus 
and Sigillariastrobus it is quite clear from Dr. Scott’s work 
that Lepidocarpon , in which the megasporangia are retained on the 
cone until after the cavity of the megaspore is filled by the pro- 
thallus, was evolved from a heterosporous Lepidostrobus. The 
general anatomy of the axis and the structure of the cone are 
practically identical with those of Lepidostrobus (15). The principal 
differences seem to be that in the megasporangium only one spore- 
mother-cell underwent the tetrad division and that only 
one megaspore came to maturity; that the prothallus was 
developed while the spore was still enclosed in the sporangium, 
though no embryo has yet been found in this stage ; and that the 
megasporangia and microsporangia were surrounded by an 
integument which grew up from the sporophyll; in the former it 
left a narrow radially elongated micropylar crevice at the top ; in 
the latter the integument was much less developed. Of the two 
species of Lepidocarpon at present known one comes from the Coal 
Measures, and the other from the Calciferous Sandstone Series 
of Burntisland, at the very base of the Carboniferous system. 
That a case of such extreme heterospory as that occurring in the 
species from Burntisland should occur at so early a period makes 
it clear that heterospory must have originated at a very early 
geological epoch, and lends some support to Dr. Scott’s view that 
all species of Lepidostrobus were heterosporous. 
All typical Lepidodendraceae disappear before the end of the 
Permian (21); but Graf Solms Laubach has described, under 
the name of Pleuromeia, some fossils perhaps allied to the order 
(22). Unfortunately Pleuromeia is only known as impressions and 
we are therefore ignorant of its anatomy. Dr. Scott has suggested 
that the four lobes found at the base of the stem of the Pleuromeia 
marked with scars left by roots may correspond to the four Stig- 
marian axes radiating from the base of the stem of Lepidodendron 
and Sigillaria (13). In its vegetative parts Pleuromeia agrees fairly 
well with the Lepidodendraceae though the whole plant is on a smaller 
scale; the casts show that the stems were medullated, the leaves 
appear to have been small, simple and peculiar, parichnos appear 
