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 HaVwieda iridens the peripheral 

 utricles show a range of from 49 to JJ fi in average maximum 

 diameter in surface view ; that is, on measuring the distal ends of 

 each the longest way, the average in some individuals is as low as 

 49 fi and in others as high as jy fi. In the representatives of H. 

 Monilc, the peripheral utricles, measured in the same way, range 

 from 30 to 44 /i in diameter ; and they are more strongly coherent 

 after decalcification, their lateral walls being in contact for \—^^ 

 their length vs. \—^-^ their length in //. tridcns. And the utricles 

 of the subcortical layer, at least those of the outmost series, are 

 narrower and less rounded than in H. tridens, being 24-55 /i in 

 greatest width vs. 35-95 />«, and obconical, clavate, or obovoid, 

 rather than turbinate, subglobose, or ellipsoidal. Halimeda tridcns 

 and H. Monde occasionally simulate each other in form, as indeed 

 in an even greater degree do Halimeda Tnna and H. scabra, and 

 it is riot 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 a 

 satisfactory basis for determination. 



In addition to Halimeda tridens, H. Monile, and //. favulosa, 

 there is a fourth member of this alliance which we have thus 

 far met with five times in Porto Rico, Jamaica, and the Bahamas, 

 and which holds its distinctive characters so constantly in varying 

 depths of water and under other diverse conditions that we believe 

 it entitled to specific rank. A description of it follows : 



