132 L. HILLIS-COLINVATIX 



This species can be fairly readily distinguished from other 

 Halimedae, except possibly cuneata, by the frequent occurrence of 

 subcuneate or obovate segments. The absence of stalk or cushion 

 segments separates it from this one species. Any confusion with 

 discoidea or taenicola can be resolved microscopically by the absence of 

 swollen secondary or tertiary utricles in lacunalis. 



Histologically, the commonness of small secondary utricles is fairly 

 characteristic of mature segments of this species. These small utricles 

 are absent, however, when the cortex is composed of only two layers. 



In other anatomical aspects lacunalis resembles tuna, particularly 

 the Atlantic and Indian Ocean specimens. Features the two species 

 have in common are the lateral adhesion of the nodal fusion units, the 

 regular surface appearance of the peripheral utricles and the apparent 

 absence of lateral fusions among them. 



Halimeda lacunalis differs from tuna in the significantly smaller 

 diameters of its peripheral utricles, with those of f. lata generally 

 towards the lower limit of the broad tuna range. 



The adherence of the nodal groups is usually fairly considerable, 

 the rigidity of the resulting nodal structure sometimes leading to 

 confusion with the Rhipsalian pattern. From this type of nodal union 

 it is, nevertheless, entirely distinct, the units in lacunalis separating on 

 teasing. Halimeda lacunalis also lacks the well-developed bulbous 

 holdfast of Rhipsalian species. 



Halimeda gigas W. R. Taylor 

 Figure 39. 



Halimeda gigas Taylor (1950), p. 84, Plate 44; Hillis (1959), p. 350, Plates 1, 

 5, 6, 9. 



Plants loosely organized, to 15 cm tall; calcification rather fight; colour 

 brownish-green, the surface sfightly glossy, rugose, tending to crack somewhat 

 on drying; branching complanate and sparse, although up to six segments 

 may arise from a single segment ; basal segments subterete, the others plane, 

 mainly discoidal to reniform, the outer margin entire or very slightly 

 undulating, to 31 mm long, 42 mm broad and averaging 0-75-1-00 mm in 

 thickness; shrinkage on drying fairly considerable, leaving a brown stain 

 on paper. 



Cortex of two, occasionally three layers of utricles; outermost utricles 

 remaining attached after decalcification for an average distance of 8 [jim, at 

 times fusing laterally in twos and occasionally threes for this same distance ; 

 unfused peripheral utricles (84-)96-130(-170) yun in surface diameter, 

 130-240 tjixn. long in section, borne two or four on each secondary utricle; 

 secondary utricles 60-120(-160) (xm. 



