Key to the species of Herposiphonia

1	Determinate branches with more than 20 segments	2
1	Determinate branches with fewer than 20 segments	3
2	Determinate branches up to 3 mm long; pericentral cells 12-14	H. pacifica Hollenberg
2	Determinate branches up to 1.25 mm long; pericentral cells 8-16	H. secunda f. tenella C. Agardh) Ambronn
3	Three determinate branches alternating with 1 indeterminate branch; occasionally a bare node between 2 determinate branches; indeterminate branches up to 130 µm in diam	H. parca Setchell
3	Three determinate branches alternating with 1 indeterminate branch; all segments with a branch; indeterminate branches up to 300 µm in diam	H. crassa Hollenberg