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 I-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, 19-654 X 11-27u, I-3 times longer than broad; 
interior cells 96-2504 X 55-110u, very thin-walled and often in- 
conspicuous when viewed through the cortex, the walls perpen- 
dicular to the surfaces mostly only I-2y thick (10—-12y in basal 
parts), collapsing on drying; rica tee forming small scat- 
tered inconspicuous sori. [PLATE 27. 
La Paz, Vives 2 (type) and Pie 
Dictyota Vives 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 1-2, thick, 
while those of D. Bartayresiana are ordinarily 3—7u thick; whether 
wholly as a result of this thinness of the walls, or in part asa 
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 Vivesti occasionally, as in most other species of the 
