Txvo Fossil Prothalli. 
3*3 
an increase to form labial ridges on each side of the line of dehi¬ 
scence, and a vague thickening somewhat further hack than the 
labia, which seems to indicate a secondary ridge surrounding the 
mouth, whatever shape, tri-radiate or otherwise, that area may have 
been. On the inside, the surface layer of the spore-wall shows 
a tendency towards becoming detached from the main portion. 
In seemingly identical spores, some of which have been found 
even in tetrad groups within a sporangial wall, the apex of the 
spore was produced into a long tubular thick-walled process, often 
twisted or flattened. 
The general shape of the spore is rather like that of a tomato 
(of which the stalk would represent the apical prolongation of the 
spore); that is to say, the vertical section shows an oval outline, 
of greater breadth than height. In conjunction with this form, the 
development of an equatorial belt of ramenta must have rendered 
it inevitable that the shorter axis of the spore should remain ver¬ 
tical, and even suggests the possibility that the spore was intended 
to float, and to germinate upon the surface of water. The appen¬ 
dages on fossil megaspores apparently acted as collectors of 
the microspores, for, in many sections, these are found entangled 
among them in large numbers. The habit of germination while 
floating may account for the production of archegonia low down 
on the prothallus, as described below. 
Within the cavity of the spore is an irregular hounding mem¬ 
brane, connected to the spore-wall at the two edges of the line of 
dehiscence, hut otherwise free. The second membrane, represented 
darker in the figure, is most probably only a crack in the matrix, 
filled with some carbonaceous material. 
The prothallus, which lies almost entirely outside the spore, 
is stretched across the open mouth, from lip to lip, and is in close 
contact with each. Although no doubt originally represented, in 
three dimensions, by a cushion of tissue, this section shows a 
bilobate outline, somewhat recalling the outline of a prothallus of 
Aspidhmi , or, more particularly, that of Salvinia, the base of the 
prothallus being in the mouth of the spore, and the lobes divergiug 
outwards. 
The prothallial tissue is formed of angular parenchymatous 
cells, rather elongated in a direction perpendicular to the surface 
of the prothallus, having thin, black walls, hut with no contents 
preserved. The periphery is marked by smaller cells than those in 
