THE GREEN AI<GAE OF NORTH AMERICA 327 



KEY TO THE SPECIES OF RHIZOCI.ONIUM. 



i. Marine. 2. 



i. Fresh water. 6. 



2. Branches at least in part similar to the axis. 3. 



2. Branches when present short and rhizoidal. 4. 



3. Forming erect tufts, branching only at the base. i. R. erect urn. 

 3. Branching not limited to base ot" tuft. 2. R. pachydermnin. 



4. Filaments 10-14 M diam.; branching not observed. 5. 



4. Filaments 15-30 fj. diam.; branches few or many. 



3. R. ripariiim. 



4. Filaments 40-70 // diam. 4. R. tortnosittii. 



5. Cells 1-2 diam. long. 5. R. Kochianiim. 



5. Cells 3-7 diain. long. 6. R. Kenieri. 



6. Branches frequent, pluricellular. 7. 



6. Branches wanting or unicellular. 8. 



7. Filaments 12-22 /*. 9. R.fontanmn. 



7. Filaments . 50-90 M. 10. R. Hookeri. 



8. Cells rarely over i '-3 diam. long. 8. R '. crisp/cm. 



8. Cells 2-8 diam. long. 9. 



9. Cell wall thin. 7. ,/?. hieroglyphicum, 



9. Cell wall thick, 6-13 /j.. n. A", crassipellitinn, var. robust inn. 



1. R. ERECTUM Collins, igoia, p. 291; P. B.-A., No. 975. 

 Tufts erect, arising from prostrate filaments 70-100 /a diam., of 

 irregular-shaped, very thick- walled cells, 1-2 diam. long, from 

 which ari,se branches either simple or once or twice forking near 

 the base, 20-50 ^ diam., cells 3-6 diam. long; branches up to 

 30 cm. long, but so much and so regularly crisped that the tufts 

 seldom exceed 10 cm. in height. In tide pools, lower literal, 

 exposed rocky shore. Me. 



Resembling a Cladophora in general appearance, but with the 

 branching limited to the base ; intermediate between the two 

 genera, but apparently nearer to Rhizoclonium . 



2. R. PACHYDERMUM Kjellman, 1877, p. 55, figs. 26 and 28. 

 Frond at first attached, main axis short, 85-100 p. diam., with 

 more or less frequent branches, 50-75 //, diam., cells 1-2 diam. 

 long ; cell wall 10-15 /* thick, lamellate ; main axis and branches 

 of first and second order set with tapering branches of few cells, 

 several times as long as their diam., with thin walls. 



Var. TENUE Kjellman, 1883, p. 310. Principal branches usu- 

 ally 30-40 n diam.; cell wall usually 5-6 /u thick; cauloid 

 branches more numerous than in the type, rhizoidal branches 

 fewer. Greenland, both type and variety. Northern fc it rope. 



3. R. RIPARIUM (Roth) Harvey, 1846-51, PI. CCXXXVIII ; 



