NO. 1 DAWSON : MARINE RED ALGAE OF PACIFIC MEXICO 145 



These are representative of the wide range of variation in size which 

 may be encountered in species of Atnphiroa and of which one must be 

 well aware in attempting determinations of these perplexing plants. 



The distinguishing features of this fiat species are seen in the abun- 

 dant basal and sparser upper branching, in the long upper segments with- 

 out midrib, and in the prominent, multizonal genicula embraced by 

 calcified projections. 



Amphiroa mexicana Taylor 



Taylor, 1945, p. 189, pi. 47; Dawson, 1949, p. 251. Amphiroa 

 peninsularis Taylor, as interpreted by Taylor, 1945, p. 188, as to Guer- 

 rero, Mexico specimens only. 



Thalli saxicolous, densely tufted, extremely brittle when dry, 3-7 cm. 

 high, consisting of many much-branched erect parts from a rather limited 

 prostrate stratum ; branching closely dichotomous, mostly at intervals of 

 1-3 segments throughout the plant, the segments quite erect, with angles 

 less than 45°; segments cylindrical to compressed, of rather uniform dia- 

 meter throughout, 0.8-1.2 mm. in diameter, or as little as 0.5 mm. at the 

 tips, 1.5-3.2 mm. long, usually 1.5-4 times as long as broad, but of ir- 

 regular length, many segments being scarcely longer than broad ; terminal 

 segments truncate, commonly with faint zonal banding; genicula rather 

 prominent, dark colored, usually coincident with forkings, commonly of 

 4 cell tiers; intergenicula with a moderately thick cortex of 3-5 layers 

 of very small cells, increasing to 6-7 layers in age and the cells elongat- 

 ing anticlinally, with a multizonal medulla showing an alternation of 

 usually 3-5 zones of long cells with 1 zone of short cells; carposporic 

 and tetrasporic conceptacles numerous, evenly distributed around the 

 branches, slightly elevated, about 250 fx in diameter, with a pore about 

 35 II in diameter flush with the surface; carpospores subspherical, 25-28 

 /A in diameter; tetrasporangia short-clavate, 40-60 yu, long, borne among 

 paraphyses from the conceptacle floor; antheridial plants not identified. 



Type: Holotype is Schmitt 120G-33 in box 3648 and on slide 

 1489 in HAHF. 



Type locality: Bahia Petatlan, Guerrero, Mexico. 



Mexican distribution: Guerrero — Taylor 568, Bahia Petatlan, 

 Mar. Oaxaca — D. 3826, Salina Cruz, Jan. 



Although few collections have as j^et been secured of this species, it 

 would appear to be abundant locally along the tropical mainland coast. 

 At Salina Cruz it was the dominant alga on all the granitoid reef rocks 



