502 Howe: PHYCOLOGICAL STUDIES 
them forms or varieties of one species and that in addition to their 
more obvious peculiarities a careful comparison under the micro- 
scope would be likely to reveal histological characters of diag- 
nostic value. We have now been able to compare microscopically 
specimens representing fifty-seven collection-numbers of these two 
“forms” from Bermuda, southern Florida, and the West Indies, 
and the results are of interest. In Halimeda tridens the peripheral 
utricles show a range of from 49 to 77 in average maximum 
diameter in surface view ; that is, on measuring the distal énds of 
each the longest way, the average in some individuals is as low as 
49 and in others as high as 77 4. In the representatives of 7. 
Monile, the peripheral utricles, measured in the same way, range 
from 30 to 44 in diameter ; and they are more strongly coherent 
after decalcification, their lateral walls being in contact for 4-5 
their length vs. 1-1, their length in H. ¢ridens. And the utricles 
of the subcortical layer, at least those of the outmost series, are 
narrower and less rounded than in H. ¢ridens, being 24-55 in 
greatest width vs. 35-95 4, and obconical, clavate, or obovoid, 
rather than turbinate, subglobose, or ellipsoidal. /alimeda tridens 
and H. Monile occasionally simulate each other in form, as indeed 
in an even greater degree do Halimeda Tuna and H. scabra, and 
it is not surprising that Lamouroux, Harvey, and others who have 
depended upon externalities have had doubts as to their specific 
distinctness ; but we believe that they are really distinct and that 
they constitute species in the best sense of the word. Ninety-five 
per cent. of the specimens that one meets with can easily be 
referred at sight to the one species or the other, and in the case of 
the few that may seem doubtful on first inspection, an accurate 
measurement of the peripheral utricles has, thus far, afforded 4 
satisfactory basis for determination. | 
In addition to Halimeda tridens, H. Monile, and H. favulosa, 
there is a fourth member of this alliance which we have thus 
it entitled to specific rank. A description of it follows : 
