NO. 1 DAWSON: MARINE RED ALGAE OF PACIFIC MEXICO 3 



9. Branching opposite or verticillate 10 



9. Branching not opposite or verticillate 14 



10. Branching opposite, or, if verticillate, with all branches of 



a verticil of about the same length 11 



10. Branching verticillate, each verticil with two long and two 



short branches Platythamnion 



11. Branching opposite 12 



11. Branching verticillate 13 



12 Tetrasporangia terminal on ordinary lateral branchlets 



Gymnothamnion 



12. Tetrasporangia abaxial or adaxial on branches or branch- 

 lets Antithamnion (in part) 



13. Thallus mucilaginous, the lateral branchlets compactly 



arranged to form a cylindrical axis Crouania 



13. Thallus not mucilaginous, the lateral branchlets loose, not 

 forming a definitive cylindrical axis .. Antithamnion (in part) 



14. Branching regularly alternate 15 



14. Branching not regularly alternate 17 



15. Tetrasporophytes with tetrasporangia 16 



15. Tetrasporophytes with polysporangia Pleonosporium 



16. Gonimoblast formed of ± spherical or rounded gonimo- 



lobes; cells plurinucleate Callithamnion 



16. Gonimoblast cordiform, or of angular or lobed gonimo- 



lobes Aglaothamnion 



17. Branching predominantly dichotomous Griffithsia 



17. Branching not predominantly dichotomous 18 



18. Plants minute; cystocarps with pericarp-like investment 



Lejolisea 



18. Plants larger ; cystocarps without loose cellular investment 19 



19. Rhizoids penetrating the host, much swollen Tiffaniella 



19. Rhizoids simple, with discoid ends Spermothamnion 



Gymnothamnion elegans (Schousb.) J. Ag. 

 PL 1, fig. 1-3 



J. Agardh 1892, p. 27; Setchell & Gardner 1930, p. 167; Taylor 1945, p. 265. 

 Callithamnion elegans Schousboe, in C. Agardh 1828, p. 162. 



Thalli consisting of a creeping axis giving rise to erect, plumose, 

 percurrent axes about 2 mm high ; prostrate axes about 20 n in diameter, 

 of cells about 1.5-2.5 diameters long, attached at frequent intervals by 

 short pluricellular, simple or branched rhizoids; erect axes percurrent, 

 15-20 fx in diameter, with a distichous pair of opposite, determinate 

 branchlets from each cell, or sometimes a conspicuous lower part of 

 the axis barren of branchlets ; determinate lateral branchlets of somewhat 

 irregular length, consisting of 6-13 cells, somewhat attenuated, usually 



