NO. 1 DAWSON: MARINE RED ALGAE OF PACIFIC MEXICO 5 



extended by the failure of Japanese algologists to deal clearly and 

 effectively with the plant treated by Okamura (1922) as Antithamnion 

 plumulum. According to his figure and description, both the distichous 

 A. plumulum var. plumulum and the tetrastichous A. plumulum var. 

 crispum occur in Japan. Inagaki (1950, p. 25) treated Okamura's 

 Japanese P. plumulum as synonymous with his P. yezoensis, but 

 without pointing out distinctions from the European plant. Furthermore, 

 Inagaki, Tokida, and Tokida & Inabu have described new Japanese 

 species of Platythamnion without providing further comparisons with 

 this incompletely tetrastichous Platythamnion-\ike plant of Okamura. 

 Indeed, Tokida has gone so far as to describe Platythamnion inter- 

 medium of which he says : "The transversal branches are usually smaller 

 than the lateral ones, but not so markedly diminished in size as in most 

 Playthamnion species. In this respect at least it cannot easily be decided 

 whether Platythamnion intermedium would belong to which of the two 

 allied genera, Antithamnion or Platythamnion." 



In view of these considerations the following key to the known 

 species of Platythamnion should be treated as an aid to their recognition 

 in relation to the closely allied Antithamnion plumulum var. crispum. 



1. Axes bearing on each cell tetrastichous branchlets in dissimilar 

 opposite pairs throughout their length from base to apex. 



(Platythamnion) 2 



1. Axes bearing tetrastichous branchlets in dissimilar opposite 

 pairs, but not on every cell, or at least not consistently all the 



way to the apex 



Antithamnion plumulum v. crispum (Ducluz) Hauck 



2. Longer pairs of lateral branchlets bearing 4 quadrately 

 arranged subultimate branchlets from each cell, at least the 



more basal ones 3 



2. Longer pairs of lateral branchlets bearing 1, 2, or 3 branches 



from each cell < 4 



3. Quadrately arranged subultimate branchlets of younger parts ± 

 symmetrical from each cell, the upper and lower pair of about 



equal size P. heteromorphum J. Ag. 



3. Quadrately arranged subultimate branchlets of younger parts 

 asymmetrical, the lower of each vertical pair much longer than 



the upper P. reversum Kylin 



4. Longer pair of lateral branchlets bearing, at least near their 

 base, 3 subultimate branchlets from each cell, 2 on the 



upper side and one on the lower P. villosum Kylin 



4. Longer pair of lateral branchlets bearing only 1 or 2 

 subultimate branchlets from each cell 5 



