8 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 26 



Type: Holotype is Dawson 379-40, Feb. 4, 1940, in Herb. Allan 

 Hancock Foundation 54. 



Type locality: Dredged in 22 meters, sandy bottom with 

 abundant worm-tubes etc. at Tepoca Bay, Sonora, Mexico. 



Mexican distribution: A single collection, D. 20528 (M. Neu- 

 shul) from a depth of 115 feet, Bahia Viscaino, seems to be this species 

 but is very small and apparently not fully developed. 



This species appears to be closely related to the Peruvian P. 

 orbigniana (Mont.) J. Ag. Well preserved specimens will be needed, 

 however, to determine the details of branching structure and reproductive 

 apparatus in order to make effective comparisons between these plants 

 from widely separated areas. An examination of part of Montagne's type 

 has revealed none of the short "dorsal and ventral" branchlets, and it 

 may be presumed, in accord with Howe's account, that these are indeed 

 "rudimentary" and incompletely developed, perhaps occurring mainly 

 in lower parts. 



Platythamnion reversum (Setch. & Gard.) Kylin 

 PI. 1, fig. 4 d-f 

 Kylin 1925, p. 54, fig. 34 d-f. Platythamnion heteromorphum f. reversum 

 Setchell & Gardner 1903, p. 345, pi. 25. 



Thalli to 2 cm in extent or more, the lower parts partially prostrate, 

 attached by numerous slender, multicellular, branched rhizoids mostly 

 arising from the basal cell of the ventral short determinate laterals, 

 consisting of an irregularly branched primary axis of cells to 300 p. in 

 diameter below and about 2 diameters long (shorter above), and giving 

 rise at each axial cell node to 4 determinate branches of two kinds: one 

 opposite pair of compound, ascending long laterals and an opposite pair 

 of much shorter dwarf laterals; long pair of determinate laterals 

 of 8-12 cells (±500 p.) long, the cells about 1.5 diameters long, 

 rigid, little attenuated, compound, with a whorl of 4 secondary branchlets 

 from each cell node, these in part with pinnate or secund ultimate 

 branchlets all sharply reduced to a point, the longer ones adaxial and 

 abaxial, and with the abaxial ones developing earliest and reaching 

 greatest development; short determinate laterals consisting of a sub- 

 spherical basal cell producing a spinose, ± radiating group of short, 

 forked, attenuate branchlets; gland cells infrequent and inconspicuous 

 on ultimate branchlets, sometimes apparently absent; tetrasporangia 

 subspherical, about 35 p in diameter, pedicellate on 1 or 2 cells replacing 

 ultimate branchlets on the long laterals; other reproduction not seen. 



Type: Not specifically designated, cited as N. L. Gardner 8 and 

 621, probably to be found in Herb. University of California, Berkeley. 



Type locality: West coast of Whidby Island, Washington. 



Mexican distribution: Gulf of Calif. — D. 21563, Isla San 

 Esteban, 1040 m. 



