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. fceniculaceus, D. 

 hippuroides, D. Chwdaria, and D. Mesoglcea being 

 British), Gdbia in the Baltic, and Scytothamnus in the 

 southern ocean exclusively. 



DESMARESTIACE.E 



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 

 (Arthrodadia) 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 



