40 L. HILLIS-COLINVAUX 



character, is rarely useful except in living specimens of scabra and 

 micronesica, which ma}^ both have a pronounced bluish cast. 



In some species the degree of calcification appears noticeably- 

 different between deep and shallow forms (Hillis, 1959; Goreau, 1963; 

 Bohm, 1973a; Section IX), and species such as fragilis, gracilis, 

 lacrimosa and hikinensis usually seem considerably more calcified than 

 cuneata or lacunalis. Finally, the shape of basal segments may be 

 noticeably different from the shape predominating in upper portions 

 of the thallus. This is most marked in micronesica where the lowermost 

 segment is usually several times larger than the other segments and very 

 irregular in outline (Fig. 46). In members of the incrassata group 

 several of the lowermost segments may remain unbranched, producing 

 a caulescent effect, or adjacent basal segments may consolidate laterally 

 forming a rather massive fan-shaped structure (Fig. 12). 



(b) Holdfast style. Appearance of the holdfast is a character which is 

 more useful in delimiting groups of species than in identifying individual 

 ones, and is correlated, to some extent at least, with type of substrate. 



Holdfasts of one group of species (Fig. 2) are conspicuous and 

 bulbous. They consist of a mass of loose filaments to which particles 

 of substrate freely adhere, so that the structure looks like cemented 

 conglomerate of sand. These holdfasts range in length from about 1 cm 

 to over 13 cm. Halimedae with these holdfasts belong to the Rhipsalis 

 section of the genus (Section IV), a group which contains all the species 

 growing on unconsolidated substrates. Thalli with such holdfasts 

 represent less than one-quarter of the described species, but as a group 

 are the most readily separated. It is possible to find Rhipsalian 

 Halimedae attached to rocks or cobbles, usually where there is a thin 

 layer of sand over the stone. The holdfast is then likely to be at the 

 small end of the size range, but is still definite and is usually clearly 

 separable from Halimedae with holdfasts of the remaining types. 



Most Halimedae are attached to a firm, generally stable substrate 

 such as coral rock. The holdfast is usually less than 1 cm long, is fre- 

 quently inconspicuous, and may be missing from the specimen unless 

 the thallus has been carefully collected. Holdfasts are commonly lacking 

 in dredged material, for example. These smaller holdfasts often appear 

 as orangey-brown, rather tightly compressed mats (e.g. tuna). The mats 

 can be peeled off the rock substrate, though the operation is delicate. 

 In some of these holdfasts the filaments remain loose so that the plant 

 appears to be attached to the rock by a random web of fine string. 



(i) Multi-holdfast species. In a few species there is a complex 

 system of attachments in place of a single holdfast. There are two 



