110 SEAWEEDS 
by the division of their apical cells, and, at the 
margin of a sorus, where they may be seen in an 
early stage of development, push off and cause to 
exfoliate the original cuticuloid gelatinous layer of the 
epidermis. In Lithoderma sporangia of both kinds 
occur in sori, but on different plants.. The plurilo- 
cular bodies, which may be presumed to give rise to 
gametes, occur as lateral off-shoots from special 
branches arising in turn from superficial cells. 
These gametangia are either a single row of cells, 
or more frequently several rows, and are more or 
less cylindrical in shape. The unilocular sporangia 
are terminal bodies—in fact transformed superficial 
cells, and are mostly obovate. The actual life-history 
has not been followed, but the relationship with 
Ralfsia cannot be doubted. The general relation- 
ships of the order are vague, but probably Chor- 
duriaceee of which Ralfsiacee may be degenerate 
allies, and Hetocarpacee (especially Ascocyclus) exhibit 
the most distinct claims. 
The Geographical Distribution extends from the 
polar seas to the tropics, and though the species 
are always few, the maximum is attained in the 
North Atlantic. The distribution of the six or 
seven species of Lithoderma is of limited range so 
far as is at present known. They occur, however, 
both in fresh-water and in the sea. The marine 
forms are found in the Arctic Sea, North Atlantic, 
and Mediterranean, while the two fresh-water forms 
occur in the south of France, Germany, and Sweden. 
L. futiseens has recently been discovered on the 
British coasts. 
