98 BRITISH SEA-WEEDS. 



some obvious bond of union in an essential, easily seen, 

 and important common character. This is found in the 

 structure of the stem in the articulated species, and of 

 the axis of the stem in species which appear to be par- 

 tially inarticulate; the dissepiments being hid by the 

 growth of a thin or thick layer of epidermal cells round 

 the stem or branches." 



The tubes, or siphons, which constitute the char- 

 acter of this genus, serve also to distinguish the vari- 

 ous species, and to enable the student to arrange these 

 in the most natural subdivisions. Recent writers on 

 the subject are not, however, agreed as to the best 

 mode of doing this; for while some have merely 

 grouped together those species which possess com- 

 mon characters, others have divided the genus into 

 two or more subgenera; and Dr. J. E. Gray, in his 

 ' Handbook of British Water-weeds,' has gone so far as 

 to separate even those species which occur on our coasts 

 into four distinct genera. The difference between these 

 various modes is not really so important as might be 

 supposed. It arises in part probably from a desire not 

 to disturb existing genera more than is absolutely neces- 

 sary, and in part from a variety of opinion as to whether 

 certain characters should be considered generic, or only 

 specific, or subgeneric. Thus we find Dr. Gray's genus 

 Polysiphonia corresponds almost exactly with the sub- 

 genus of the same name iu Dr. Harvey's ' Nereis Bo- 

 reali- Americana ;' and so also the genus Oligosiphonia, 

 of the former author, with the subgenus of the same 

 name of the latter. Leaving higher authorities to de- 

 termine which is the best arrangement, I shall merely 

 group together the allied species, and thereby avoid the 

 use of names not hitherto known to collectors of Bri- 

 tish Sea-weeds. 



