88 L. HILLIS-COLINVAUX 



9. Peripheral utricles generally exceeding 50 [jtm in surface diameter; 

 segments cylindrical to cuneate to discoidal and reniforra with the 

 outer margin often deeply lobed ; plants to 24 cm tall H. incrassata, p. 93 

 10. Basal holdfast region appearing to involve several of the lower- 

 most segments which consequently may be obscured by particles 

 of sand adhering to the rhizoidal filaments of the region ; entire 

 holdfast area may spread laterally for 1 cm or more . . . . 11 



10. Basal holdfast region restricted to a single segment, holdfast 

 ranging from relatively conspicuous and about 1 cm in greatest 

 dimension to almost negligible, any adhering particles of loose 

 substrate usually can be scraped away readily to show a localized 

 holdfast 12 



11. Area of basal segments of thallus generally two or more times 

 larger than those of rest of thallus so that a pronounced stalked 

 effect is produced . . . . . . . . H. melanesica, p. 153 



11. Segments of fairly uniform size (area) throughout, with maximum 



size approximately 4 mm long, 5 mm broad . . H. renschii, p. 115 



12. Growth pattern often forming laterally spreading thalli to 25 m or 

 more which may be accompanied by some erect growth, or growth 

 pattern mostly erect producing densely branched clumps; 

 branching often multidirectional so that thallus does not lie flat; 

 holdfast attachment generally in several places rather than by a 

 single basal holdfast attachment although these small attach- 

 ments may be lost during collecting . . . . . . . . 13 



12. Other than the above . . .. .. .. .. .. .. 15 



13. Branching commonly in one plane so that thallus lies relatively 

 flat; segments generally flat but sometimes arched, commonly 

 white, glossy, broadly subcuneate, sometimes trilobed; cortex 

 generally of two layers of utricles with secondary utricles slightly 

 to distinctly clavate at apical end; secondary utricles supporting 



four to eight peripheral utricles . . . . . . H. gracilis, p. 144 



13. Branching commonly multidirectional so that thallus does not lie 

 flat; segments flat or contorted, their surfaces dull; cortex 

 generally of three or more series of utricles which are not expanded 

 at the apical end and which bear only four peripheral utricles at 

 most . . . . . . . . . . . . . . . . . . 14 



14. Segments relatively large, to 16 mm long, 19 mm broad; often 

 contorted; peripheral utricles measuring 36-60 fxm in surface 

 diameter . . . . . . . . . . . . H. distorta, p. 120 



14. Segments smaller, plane or somewhat contorted, to approximately 

 7 mm long, 11 mm broad; peripheral utricles measuring 12^1 yun 



in surface diameter . . . . . . . . . . H. opuntia, p. 110 



15. Basal segment flabellate (Fig. 46), to approximately 12 mm long, 



18 mm broad, supporting numerous branches H. micronesica, p. 149 

 15. Basal segment not flabellate . . . . . . . . . . 16 



