498 Howe: Phycological studies 



dulate; main segments oblong or obcuneate, 3-8 mm. broad, 

 diminishing conspicuously in length and width in the apical sub- 

 divisions, the terminal segments mostly 1-2 mm. wide, their 

 dichotomies forming angles of about 45 , the apices obtuse or 

 subacute; surface on drying mostly smooth or slightly and ir- 

 regularly reticulate in the older parts ; cortical cells nearly uniform 

 in size and color, 19-65^ X 11-27^, 1-3 times longer than broad; 

 interior cells 96-250/z X 55-110,11, very thin-walled and often in- 

 conspicuous when viewed through the cortex, the walls perpen- 

 dicular to the surfaces mostly only 1-2/1 thick (io-12/z in basal 

 parts), collapsing on drying; tetrasporangia forming small scat- 

 tered inconspicuous sori. [Plate 27.] 

 La Paz, Vives 2 (type) and i8e. 



Dictyota Vivesii is perhaps most nearly related to D. Bartay- 

 resiana Lamour., but cannot well be identified with that West 

 Indian species. It is more cespitose in habit of growth than D. 

 Bartayresiana, more stupose at the base, less regularly dichotomous 

 towards the apices, rather broader in its broadest parts and more 

 conspicuously dwindling in width as the ultimate segments are 

 approached, the axils (the upper at least) are more acute and the 

 segments less patent or divaricate, the apices are less acute, and 

 both the cortical and the interior cells are for the most part nar- 

 rower and the cortical cells overlying the septa and lumina of the 

 interior cells show scarcely any of that differentiation in form and 

 translucency that led J. Agardh to describe D. Bartayresiana as 

 "fenestrate"; the walls of the interior that are perpendicular to 

 the surface are remarkably thin, being usually only \-2y. thick, 

 while those of D. Bartayresiana are ordinarily 3-7/i thick; whether 

 wholly as a result of this thinness of the walls, or in part as a 

 result of the treatment the specimens may have received, we do not 

 know, but the interior cells of our dried specimens of D. Vivesii are 

 so completely collapsed that they do not at all regain their natural 

 form on being soaked with water, though they revive tolerably 

 well on being treated with a solution of potassium hydrate. The 

 smoothness of most parts of the surface of D. Vivesii, in its dried 

 state, may be due in some measure to the presence, in many por- 

 tions of the thallus, of a layer of small oval diatoms so closely 

 adherent and so evenly disposed that their presence is revealed 

 only by the higher powers of the microscope. The tetraspores 

 of Dictyota Vivesii occasionally, as in most other species of the 



