NO. 1 DAWSON : MARINE RED ALGAE OF PACIFIC MEXICO 135 



7. Segments 1.5-2.0 mm. wide, 2.5-4 mm. long .... 



A. brevianceps 



7. Segments 1.5-2.5 mm. wide, 5-15 mm. long .... 



A. magdalenensis 



8. Genicula usually readily visible from the surface . . 9 

 8. Genicula obscure, mostly not visible from the surface . . 



A. drouetii (in part) 



9. Segments cylindrical to compressed, 1.5-4 times as long as 



wide, not commonly forked . . . . A. mexicana 



9. Segments compressed to flattened, more than 4 times as long 



as broad (at least the simple segments), frequently forked . 10 



10. Plants erect; segments 0.7-1.2 mm. in diameter; mostly 



over 10 diameters long A. zonata 



10. Plants erect or decumbent; segments mostly less than 0.6 

 mm. broad (except at expanded points of forking), less 



than 8 diameters long 11 



11. Compressed or flattened throughout; forked segments much 

 expanded distally; apices more or less expanded, rounded 



A. crosslandii 



11. Uppermost segments cylindrical or nearly so; forked segments 

 not particularly expanded ; apices truncate or blunt 

 A. franciscana and var. 



Amphiroa foliacea Lamouroux 



Lamouroux, 1824, p. 628, pi. 93, figs. 2-3; Taylor, 1945, p. 192, pi. 

 56; Weber van Bosse, 1904, p. 92, pi. 14, fig. 1-11. 



Thalli saxicolous, about 2 cm. high or less, consisting of several sub- 

 erect or partially decumbent branched parts from a small, crustose 

 stratum; branching irregularly dichotomous to trichotomous, more or 

 less in one plane, the angles rather narrow or widely divaricate; seg- 

 ments dimorphic, compressed to flattened, simple or irregularly once or 

 twice forked, narrow, 1 mm. or less broad, 2-5 times as long as wide, 

 with inconspicuous, narrow, thin, wing-like margins, or to 3 mm. broad 

 and the wings expanded with undulate margins and the thick midrib 

 emphasized, especially on the under side; terminal segments blunt, 

 similar to the lower ones, or variously irregularly reduced or narrowed ; 

 genicula prominent, not occurring regularly at the forkings, consisting 

 of a cortical layer and a multizonal medulla of 2-4 tiers of long cells, 

 about as broad as the intergenicular medulla; intergenicular medulla 



