NO. 1 DAWSON: MARINE RED ALGAE OF PACIFIC MEXICO 7 



cells abundant both on long and short determinate laterals, on ultimate 

 or subultimate branches, lying on a single cell, sometimes serially on 

 adjoining cells ; tetrasporangia ovoid, 30-40 p. long, sessile or pedicellate, 

 usually in a group on the lower compound branchlets of determinate 

 laterals; spermatia in small adaxial pedicellate clusters on (or replacing) 

 branchlets near the base of determinate laterals. 



Type: Not specifically designated: represented by several syntypes 

 probably in the Agardh Herbarium, Lund, Sweden. 



Type locality: Not specifically designated: Friday Harbor, 

 Canoe Island, and Peavine Pass [Puget Sound, Washington], dredged at 

 5-10 fathoms. 



Mexican distribution: Pacific Baja Calif. — D. 8268, 8472 

 (70-80 m.), D. 8431a (50-60 m.) Isla Guadalupe. Gulf of Calif.— D. 

 6837 (20 m.) Bahia San Lucas. Revillagigedo Arch. — A.H.F. Sta. 

 135-34, Jan. 5, 1934, Bahia Sulphur, Isla Clarion, (50 m.) 



Reexamination of liquid isotype material of Taylor's Antithamnion 

 pectinatum var. laxa shows this to be a somewhat lax and attenuated 

 form represented in our collections from Isla Guadalupe. D. 8431 from 

 that locality represents an extremely attenuated form of this in which 

 the determinate laterals become somewhat flagellate and to 16 cells long. 



The close relationship between this species and the European 

 Antithamnion plumulum var. crispum has been pointed out above; 

 indeed, one collection, D. 8267 from Isla Guadalupe is here assigned 

 to Antithamnion plumulum (see below) because of its infrequent or 

 only occasional production of short, spine-like determinate branchlets 

 in a plane at right angles to the regular distichous laterals. 



Platythamnion tepocensis Daws. 



PI. 2 

 Dawson 1944, p. 314, pi. 72, fig. 1 



Thalli to 3 cm high or more, the attachment unknown, consisting 

 of irregularly branched primary axes 50-70 /x in diameter above, to 

 150 p. in diameter below, giving rise af each axial cell node to a whorl 

 of four determinate branches of two kinds: one opposite pair of com- 

 pound ascending long laterals and an opposite pair of relatively simple 

 dwarf laterals; cells of main axis mostly two diameters long except the 

 shorter basalmost one, attenuated, the tips acute, simple or commonly 

 producing both adaxially and abaxially simple pinnae; short determinate 

 laterals simple or consisting of 2 or 3 spreading branchlets about 60-100 

 fx long from a rounded basal cell; gland cells frequent on determinate 

 laterals, lying along a single cell; spermatangia replacing some of the 

 opposite, alternate branchlets of the long laterals, consisting of 2-3 cells 

 producing a short cylindrical cluster of spermatia; other reproduction 

 unknown. 



