NO. 1 DAWSON : MARINE RED ALGAE OF PACIFIC MEXICO 105 



tidal, Nov. Revillagigedo Archip. — ^Taylor 39-35, 34-62, Isla Clarion, 

 Jan.-Mar. ; Taylor 34-40B, Isla Socorro, Jan. Nayarit—D. 3708, Mira 

 Mar, intertidal, Dec. 



The collections cited above are from dredge hauls unless otherwise 

 indicated. Although the plant has been taken from depths as great as 

 100 meters, it may occur in sheltered intertidal localities if shaded from 

 strong light. 



Our material is assigned to this apparently widespread species follow- 

 ing the work of Weber van Bosse who examined Greville's t}^pe and 

 compared it with her East Indian specimens. Like the East Indian, the 

 Mexican plants here identified with her subspecies orientalis differ from 

 the Mediterranean type mainly in the absence of multicellular rhizoids 

 and in the less prominent lobing of the margins. Although the rhizoid 

 character would appear to be suitable for specific segregation, Weber 

 van Bosse reported some variability in the prevalence of multicellular 

 rhizoids in the Mediterranean P. rubra and felt it better to recognize 

 the Pacific examples only in a subspecific category. The Mexican plants 

 may be distinguished from other Peyssonelia species by their relatively 

 thin, lightly calcified, loosely attached thalli, their usually erect, rather 

 than oblique, perithallial cell rows, and by their elevated, more or less 

 gelatinous tetrasporangial sori. 



Peyssonelia conchicola Pice. & Grun. 

 Plate 11, figs. 12-13 



Piccone & Grunow, in Piccone, 1884, p. 317, pi. 7, figs. 5-8; Weber 

 van Bosse, 1921, p. 272, fig. 91. 



Thalli crustose, more or less orbicular, 2-3 cm. broad, growing on 

 shells and nullipores, dull, pale reddish in color, firmly attached to the 

 substrate by numerous short, unicellular rhizoids and by a heavily cal- 

 cified lower surface, with only the margins somewhat free, lightly cal- 

 cified over all, 70-110 jx thick in sterile parts; radial transection showing 

 a distinct hypothallus of large, rectangular cells broader than tall, 20- 

 26 ju, broad, 11-16 /a tall, giving rise to oblique or ascending, once or 

 twice branched perithallial cell rows in which the lower cell resembles 

 the hypothallus cell, the 2-3 middle tiers are taller than broad, and the 

 upper 2-4 tiers become progressively shorter to 5 ;ia high and about 11-12 

 ju. broad; carposporangial nemathecia forming dark, orbicular or elon- 

 gated spots on the surface, 1-3 mm. in diameter, strongly elevated, 

 gelatinous, to 120 ju, thick, of slender, non-clavate, mostly unbranched 



