146 L. HILLIS-COLINVAUX 



utricles usually remaining slightly attached on decalcification, occasionally 

 separating but still appearing hexagonal rather than rounded in surface 

 view, 23-58(-70) [im in surface diameter, 40-90(-110) [xm long in section, 

 up to eight supported by each secondary utricle ; secondary utricles clavate, 

 23-70(-125) [j,m broad, usually relatively long and frequently extending to 

 the medulla. 



Nodal medullary filaments fused most commonly in twos but occasionally 

 in threes, fusion complete; fusion groups not entangled and at most adhering 

 only slightly. 



Type specimen. Harvey's Ceylon Algae No. 72; isotype material 

 in several herbaria including BM, NY, 



Habitat. This species, which is often straggling and sprawling 

 frequently grows at the bases of coral rock and therefore may be 

 associated with sand as well as rock. Attachment is provided by a 

 multi-holdfast system. At Enewetak Atoll gracilis was most 

 commonly associated with distorta and macrophysa, occasionally 

 with minima. The known vertical range for the species is — 1 m 

 to -70 m. 



Geographical distribution. Pantropical; western and eastern 

 Indian Ocean; western Pacific, both north and south; western 

 Atlantic, both north and south. 



This species seems closely related to lacrimosa and probably also to 

 bikinensis. From bikinensis, a taxon of relatively compact specimens 

 with large segments, to 16 mm long and 25 mm broad, gracilis is often 

 readily distinguished by its loose straggling habit which reaches a length 

 of 1 m or more, and by its smaller segments, to 9 mm long and 15 mm 

 broad. In contrast, plants of lacrimosa reach about 5 cm in length and 

 are composed of spherical or tear-shaped segments rather than flat ones. 



Although gracilis is pantropical, its known distribution is extremely 

 limited as compared to that of discoidea or opuntia. The paucity of 

 records can be partly attributed to the relatively frequent misidentifica- 

 tions of material of this species as opuntia, and less commonly as 

 incrassata or tuna. Identity problems can be resolved microscopically 

 because nodal medullary filament pattern and appearance of the 

 cortical layers are both distinctive for gracilis. 



The peripheral utricles in plants of this species may initially appear 

 rounded in surface view and give the impression of being separate. A 

 more careful examination, however, usually reveals that they are at 

 least slightly attached, though often by only the thin platform-like 

 extensions of the outer surface. The different optical properties of these 

 edges and the slightly convex outer face of the utricles combine to 



