122 L. HILLIS-COLINVAUX 



Habitat. A spreading alga, growing on rock, in and through its 

 crevices, and around the bases where it may extend into sand. 

 Attachment is provided by a multi-holdfast system. This species 

 grows in regions of strong currents such as the inter-island passes 

 and quarry at Enewetak. The limited depth records are all from 

 water of — 0-6 m to —3 m. 



Geographic distribution. Western Pacific Ocean, both north and 

 south. 



3. Section Halimeda J. Ag. ex De Toni 



Halimeda tuna (Ellis and Solander) Lamouroux 



Figure 35. 



Corallina tuna Ellis and Solander (1786), p. Ill, Plate 20, Fig. e. 

 Halimedea tuna Lamouroux (1812), p. 186; Halimeda tuna Lamouroux (1816), 



p. 309, Plate 11, Fig. 8; Barton (1901), p. 11, Plate 1, Figs 1-6; Collins 



(1909-1918), p. 400; Taylor (1928), p. 85; Hillis (1959), p. 342, Plates 1, 



5, 6, 9. 

 ''^ Halimeda tuna f. albertisii Piccone (1879), p. 23, Fig. 2; Barton (1901), 



p. 14, Plate 1, Fig. 3. 

 Halimeda tuna f. platydisca (Decaisne) Barton (1901), p. 14, Plate 1, Fig. 2; 



Halimeda platydisca Decaisne (1842), p. 102. 

 Halimeda tuna v. platydisca Borgesen (1911), p. 134; Borgesen (1913), p. 106, 



Collins (1909-1918), p. 102; Taylor (1928), p. 85, Plate 10, Fig. 13. 



Plants spreading or compact, often forming cushion-like clumps, arising 

 from an inconspicuous holdfast, to 15 cm tall; calcification moderate to light, 

 sometimes becoming heavier towards the base; branching generally ditri- 

 chotomous; basal segments often subcuneate, the others plane or sometimes 

 ribbed, mainly subcuneate, discoid or reniform, commonly relatively small, 

 to 13 mm long, 19 mm broad, and averaging 0-50-0-75 mm in thickness, but 

 in other specimens, often from deep water, to 25 mm long and 40 mm broad. 



Cortex of two to four layers of utricles; outermost utricles remaining 

 firmly attached after decalcification for a distance of approximately 

 12-16 (j.m, sometimes fusing laterally in twos or threes for this same distance ; 

 lateral and peripheral walls occasionally somewhat thickened; unfused 

 peripheral utricles measuring (25-)34-100(-125) y.m in surface diameter, 

 (46-)60-130(-230) jam long in section, two to four or occasionally up to 

 seven supported by each secondary utricle; secondary utricles (20-)30- 

 80(-110) [Jim broad; tertiary utricles 40-130 [jim broad. 



Nodal medullary filaments uniting in twos and threes, the length of fusion 

 usually 1-5 times the filament diameter or else complete; resultant units 



