NO. 2 DAWSON : MARINE RED ALGAE OF PACIFIC MEXICO 275 



transection, but this is mainly due to difference in the length of the 

 anticlinal medullary filaments in response to the environment. The 

 refractive filaments of the medulla with their stellate "ganglia" are 

 usually prominently visible through the thin cortex. 



Reexamination of the type of Halymenia refugiensis has revealed that 

 it is an antheridial plant whose structure satisfactorily agrees with that 

 of H. actinophysa. 



Differences from the superficially similar Halymenia megaspora have 

 been pointed out under that name. 



The collections from Bahia Agua Dulce, Isla Tiburon represent a 

 form with prominent, simple or branched stipes, but which structually 

 and reproductively does not seem distinguishable from other collections 

 of this species. 



Key to the Mexican species of Zanardinula 



1. Secondary pinnate branches indeterminate, 3.5-15 mm. broad 2 

 1. Secondary pinnate branches usually determinate, 3.0 mm. broad 



or less 3 



2. Secondary branches relatively few, 5-15 mm, broad, to 20 

 cm. long; tetrasporangia in extensive nemathecial sori on 



secondary blades Z. andersoniana 



2. Secondary branches abundant, becoming 3.5-6 mm. broad, to 

 9 cm. long; tetrasporangia unknown . Z. lyallii var. ? 

 3. Primary or major branching clearly dichotomous .... 4 



3. Primary or major branching irregular 8 



4. Thalli under 4 cm. high; segments 0.5-1 mm. broad 



Z. acroidalea 



4. Thalli 6-25 cm. tall; segments 1-3 mm. broad ... 5 

 5. Branches 2-3 mm. broad ; secondary branches short, mostly less 



than 3 mm. long 7 



5. Branches about 1 mm. broad . 6 



6. Thalli low, spreading; secondary branches ±: indeterminate, 



1-1.5 cm. long . . Z. abbreviata var. guaymasensis 



6. Thalli erect; secondary pinnate branches ± determinate, 



3 mm. long or less Z. vizcainensis 



7. Young, growing branches ± spatulate; thalli usually under 



8 cm. high Z. abbreviata var, abbreviata 



7. Young, growing branches not spatulate; thalli usually over 

 12 cm. high Z. cornea 



