NO. 1 DAWSON : MARINE RED ALGAE OF PACIFIC MEXICO 143 



Thalli saxicolous, forming rounded clumps 2-3.5 cm. high, consisting 

 of a number of branched erect parts from the crustose stratum, not very 

 brittle when dry; branching rather regularly and closely dichotomous, 

 more or less in one plane; lowest segments subcylindrical or compressed, 

 1.5-2.0 mm. in diameter, scarcely longer than broad, coarse, angular, 

 those above grading quickly into the normally flat, expanded upper seg- 

 ments which are 1.5-2.0 mm. broad and 2.5-4 mm. long (to 3 mm. broad 

 at the distal end initiating a fork), the margins somewhat irregularly 

 undulate and thinner than the middle, but without a midrib; inter- 

 genicular medulla showing an alternation usually of 3 tiers of long cells 

 and 1 tier of short cells; genicula very prominent, dark colored, normally 

 coincident with the forkings, less than half as broad as the contiguous 

 calcified portions of the segments above and below, and between which 

 they form a broadly elliptical, uncalcified "window," multizonal, of 

 several tiers (up to 10) consisting only of medullary type cells with the 

 same alternation as in the intergenicula; tetrasporic and carposporic 

 conceptacles abundant, scattered over both faces of intergenicula, form- 

 ing rather prominent, blister-like elevations 300-400 ju, in diameter, the 

 small ostiole usually clearly evident ; antheridial plants not seen. 



Type: Holotype is Dawson 3825, Jan. 10, 1947, on sheet 55180, 

 including vial 2259 and slide 1499, in HAHF. 



Type locality: On rocky shore just east of Salina Cruz, Oaxaca, 

 Mexico. 



Additional material: Two other rather scant collections seem- 

 ingly of this species appear to indicate a rather wide range for this plant 

 in Mexico; D. 593, near Guaymas, Sonora, Feb.; D. 6895, Punta 

 Frailes, Baja Calif., March. 



The distinctive characters of this species are the short, broad, flat 

 segments without midrib, the dichotomous-flabellate branches, and the 

 prominent, "embraced" genicula. In several respects it is similar to 

 Amphiroa anceps of the southwestern Pacific, but as originally described 

 by Lamarck (1815, p. 238) and as illustrated by Weber van Bosse 

 (1904) and by Harvey (1847), the segments of the latter are elongate- 

 ancipitous, mostly 4-5 times as long as broad. 



Amphiroa magdalenensis sp. nov. 

 Plate 30, fig. 2 



Thallis laxe rosulatis, ad 5 cm. altis; segmentis complanatis praeter 

 ad basim; segmentis cylindricis, infimis, brevibus complures ramos feren- 

 tibus; ramificatione superiore exigue dichotoma vel trichotoma in una 



