Two Fossil Prothalli. 
3 i 8 
exterior to the spore, and resembling strongly the position of the 
corresponding organs in such Hydropterideae as Pilularia, can only 
have been attained, if the mature structure was developed from a 
small cap of cells, by the upward pressure of a mass of material 
originally occupying the interior of the spore (whether undif¬ 
ferentiated, plasmatic reserve-material, or a firm cellular tissue), 
and further, by continued growth in thickness of the original cell-cap, 
until the protruding cushion was produced. There is a distinct 
presumption, from analogy with corresponding specimens, that the 
whole interior was actually cellular in nature, but this cell-structure 
has disappeared, possibly subsequent to the invasion by fungus 
hyphae. ' 
In Text-fig. 2 are shown some other similar prothalli for 
purposes of comparison. 
Salvinia in younger stages shows the lobate outline of the 
present specimen. Mnzocarpon cannot be very well taken into 
comparison, as it has never been described in a fully developed 
condition. 
The figure given by Gordon of the prothallus of Lepidostrobus 
Veltlieimianns in the Trans. Bot. Soc., Edinburgh, 1908, resembles 
in general character that of Mnzocarpon figured above. Isoetcs on 
the other hand is more reminiscent of SelagineUa. 
To sum up, it may be said that this specimen represents a stage 
in the reduction of the primitive free-living Lycopod gametophyte, 
toward the condition obtaining in the “ seed ” of Lepidocnrpon. The 
prothallus was not produced until after the megaspore had been shed. 
It developed outside the spore, but remained attached to the spore- 
wall at its base, and in form resembled the prothalli of modern 
heterosporous ferns. 
The gametophyte of Dothrodcndron was then more primitive 
than that of Lepidodeudron, where the archegonia developed inside 
the spore. 
My best thanks are due to Professor F. W. Oliver, F.R.S., who 
suggested this description for the use of both the above specimens 
and for his kindness in revising the manuscript; also to Professor 
A. C. Seward, F.R.S., for much kind interest and information; and 
to Mr. D. S. Watson for his identification of the spore. 
Plates V and VI, illustrating this paper, are reproduced in 
photogravure, from burnt umber drawings by the author, by the 
Swan Electric Engraving Company, Ld., and represent with extreme 
faithfulness both the form and the colour of the original sections. 
