NO. 1 DAWSON : MARINE RED ALGAE OF PACIFIC MEXICO 141 



Thalli saxicolous, densely tufted, forming compact, pulvinate clumps 

 to 2.5 cm. high and 5 cm. broad; branching dichotomous and more or 

 less decussate, the intervals mostly 2-3 mm.; branches cylindrical to 

 compressed, 350-550 /a in diameter, a little broader at the forks, rigid, 

 apparently unsegmented, with rather faint, zonal bands; apices truncate 

 or blunt; genicula mostly concealed by calcification, widely separated 

 (the intergenicula 1-2 or even 3 times forked), in lower parts frequently 

 evidenced by a crack in the calcification, but such a crack in younger 

 parts readily recalcified, usually of 2-3 tiers of non-calcified, long medul- 

 lary cells surrounded by a layer of normally calcified medullary cells and 

 the cortex; medulla usually showing an alternation of 3-5 tiers of long 

 cells and 1 tier of short cells ; cortex thin, of only about 2 layers of small 

 cells; tetrasporic conceptacles borne on branches deep within the clump, 

 semi-superficial, aggregated on all sides of segments, domoid, 200-250 fju 

 in diameter ; sexual plants not seen. 



Type: Holotype is Dawson 717-40, July 18, 1940, in box 3531 

 and on slide 1502 in HAHF. 



Type locality: Intertidal on reef at north end of Isla Turner, 

 off Isla Tiburon, Sonora, Mexico. 



Additional material: Drouet & Richards 3329, intertidal pools 

 in the cove north of Cabo Arco, near Guaymas, Sonora, Dec. 19, 1939. 



This peculiar species is outstanding because of the obscurity of the 

 genicula which are entirely hidden within a calcified layer both of corti- 

 cal and of medullary cells. The densely compacted clumps and the near- 

 ly cylindrical segments make it readily recognizable although it resembles 

 somewhat in gross appearance some of the smaller, more densely tufted 

 forms of Amphiroa zonata. 



Amphiroa dimorpha Lemoine 



Lemoine, 1929, p. 76, pi. 3, fig. 3-4, pi. 4, fig. 6, text fig. 33; Taylor. 

 1945, p. 192, pi. 55. Amphiroa pusilla Yendo, as interpreted by Dawson, 

 1944, p. 276 in part. 



Thalli forming low, coarse, semi-rosette groups on nullipores or 

 corals, to 2 cm. or more high or brbad, suberect or commonly the 

 branches becoming repent and more or less appressed to the substratum, 

 consisting of several closely spaced, coarse, very short, subcylindrical or 

 compressed basal segments from the crustose stratum, very irregularly 

 dichotomously branched to give rise to successively more flattened and 

 expanded upper segments ; upper segments commonly extremely irregular 

 in size and shape, simple or commonly forked or lobed, to 3-4 mm. wide 



