NO. 1 DAWSON: MARINE RED ALGAE OF PACIFIC MEXICO 9 



Taylor's Platyth amnion reversum var. laxum from the Galapagos 

 Islands lacks the distinctive characteristic of P. reversum, namely the 

 4-ranked branchlets of the determinate laterals, and seems much more 

 closely related to P. orbigniana from Peru, to which it should be referred 

 for more critical study. 



Platythamnion heteromorphum J. Ag. 



PL 4, fig. 4 

 J. G. Agardh 1892, p. 23; Smith 1944, p. 315, pi. 79, fig. 3. 



Thalli to 2.5 cm in extent, consisting of a primary distichous axis 

 with indeterminate branches at intervals of about 4 cells, attached by 

 branched, multicellular rhizoids from lower decumbent parts of the 

 axis; primary axis about 150-200 /x in diameter, of cells 1.5-2.0 dia- 

 meters below, 1.0-1.5 diameters long above, provided near the distal end 

 of each cell node with a whorl of 4 determinate branches: one opposite 

 pair 400-600 /x long, the other dwarf and consisting only of a few 

 branchlets to 60 fx long from the basal cell; long determinate laterals of 

 12-16 cells mostly 1.5 diameters long, each provided with an opposite 

 pair of equal, symmetrical secondary branchlets on the adaxial and 

 abaxial sides, these in turn pinnately branched; all ultimate branchlets 

 acute; gland cells mostly on the ultimate branches, lying on a single 

 cell; tetrasporangia subglobose, 28-30 /x long, 24-27 /x broad, born on 

 the pinnate branchlets of the long laterals; other reproduction not seen. 



Type: A collection by C. L. Anderson in the Agardh Herbarium, 

 Lund, Sweden. An isotype slide is in Herb. Allan Hancock Foundation. 



Type locality: Santa Cruz, California. 



Mexican distribution: Pacific Baja Calif. — D. 7978, Cortez 

 Bank, 40-50 m.; D. 8448, Isla Guadalupe, 95 m. 



This is a readily recognized, distichous species which appears 

 complanate because of the prominant, symmetrical development of the 

 long laterals and the very limited development of the short laterals. 



Platythamnion villosum Kvlin 

 PL 3, fig. 1; PL 4, fig. 1 



Kylin 1925, p. 51, fig. 32 a-b, fig. 33 ad; Smith 1944, p. 315, pi. 79, fig. 2; 

 Dawson 1944, p. 342. 



Thalli to 6 cm in extent or more, consisting of an irregularly 

 branched primary axis to 150-300 \x in diameter, the branches alter- 

 nating at intervals of 3-8 cells, giving rise at each axial cell node to a 

 whorl of 4 determinate branches of 2 kinds: one opposite pair of 

 ascending long laterals and an opposite pair of short laterals; cells of 

 main axis about 2 diameters long, shorter above, the determinate 

 laterals arising from near the distal end; long pair of determinate 

 laterals 6-9 cells long, each cell except the terminal ones bearing 3 simple 

 or compound branchlets, 2 on the upper side, and one on the lower 



