54 L. HrLLIS-COLINVAUX 



Extent of the cortex also varies with other factors including age 

 and location in the segment. The number of layers is generally fewer 

 in young segments, and greater in old basal segments which are often 

 greatly thickened. Their number frequently is reduced or they may be 

 absent in the vicinity of the node (Fig. 6), with peripheral utricles then 

 being supported directly by short, unmodified branches of the medullary 

 filaments. And, in cryptica, a Caribbean species found often in heavily 

 shaded crevices at depths of about 25 m or greater (Colinvaux and 

 Graham, 1964), and which grows more openly in the reef to 100 m 

 (Moore et al., 1976), the cortex of the under (umbral) surface is 

 frequently somewhat atrophied. Asymmetry in cortical development 

 between the two surfaces of a segment is not conspicuous in typical 

 thalli of the other known Halimeda species. 



B. The genus and its sections 



Although the pattern of nodal filaments can no longer be relied on 

 almost exclusively for separating the species of Halimeda as Barton 

 chose to do, it is helpful and reliable for recognizing groups of species. 

 I became increasingly aware of the significance of this feature, as a 

 kind of "master character", while working on my 1959 revision 

 (Hillis, 1959), and in it referred to tuna and incrassata complexes. From 

 continued herbarium studies, combined with more extensive field work, 

 I have found that separation along these lines not only seems valid 

 on the basis of phylogenetic implications, but also that each of these 

 patterns of nodal filaments is frequently accompanied by other 

 similarities. This seems especially true of a number of habit characters, 

 and is well illustrated by species with the incrassata type of fusion 

 (Table III, filaments fuse into a single unit), all of which possess a large 

 bulbous holdfast and grow erect, or essentially so, in an unconsolidated 

 substrate. These groups appear to represent natural units within 

 Halimeda which should be recognized formally. 



A meagre framework for sections within the genus exists in the 

 extensive publication on algal systematics by Agardh (1887), in which 

 four subcategories, Tunae, Pseudo-Opuntia, Opuntiae and Rhipsales, 

 are indicated (Table IV). De Toni (1889) called them sections and 

 applied Agardh's descriptions to them. Since appearance of the segments 

 is the major character used, it perhaps is not surprising that the 

 categories, as delimited, do not seem meaningful or useful. They were 

 not even mentioned a few years later by Barton (1901). Indeed, they 

 disappeared from use with De Toni (1889). Agardh's framework can 

 be developed, however, into a modern system of categories delimiting 

 various species groups. 



