PH AOPHYCEA 99 
that no sure indication of its relationship is thus 
afforded. 
The Geographical Distribution is in north and 
south temperate seas, extending to the Arctic at 
least. Dictyosiphon occurs in both the North 
Atlantic and southern ocean (D. feniculaceus, D. 
hippuroides, D. Chordaria, and D. Mesoglea being 
British), Gobia in the Baltic, and Seytothamnus in the 
southern ocean exclusively. 
DESMARESTIACEA 
General Characters—The thallus consists of a 
primary branching row of cells with a growing point 
below the apex; beneath this growing point a mantle 
of tissue is produced which clothes the primary cell- 
row and eventually forms a thallus of considerable 
thickness. Its growth in length is maintained by 
the subterminal growing point above this secondary 
mantle of tissue. Unilocular sporangia are formed 
(Desmarestia) by the differentiation of superficial cells 
below the growing point; and plurilocular sporangia 
(Arthrocladia) from rows of cells. 
The Thallus is usually of considerable stature, 
especially in the species of Desmarestia. Two sorts 
of branches are formed—long ones, and, more abun- 
dantly, short ones. They are generally in two rows 
and mostly alternate, but sometimes opposite. The 
primary row of cells forming the thallus produces 
branches abundantly, and these as well as the main 
axis grow in length by the repeated transverse 
H 2 
