ECOLOGY AND TAXONOMY OF Halimeda 53 



It is the tertiary utricles of taenicola that are swollen and distinctive 

 (Fig. 20, No. 14), rather like the secondary utricles oi discoidea (Taylor, 

 1950; Hillis, 1959). 



Length of the inner utricles in some species varies considerably with 

 the age of the material or the number of layers of utricles. It is therefore 

 generally not a helpful character. 



(f ) Pattern and extent of inner cortex. For many species the distinctive- 

 ness of the inner cortex lies as much in the pattern formed by the inner 

 utricles (inner cortex) as a whole, as in the initial distinctiveness of 

 secondary or tertiary utricles. The pattern may be observed in sagittal 

 sections (Fig. 20). 



There are two general patterns of development of the cortex 

 (Hillis, 1959, and Fig. 20). 



The branching and rebranching of the lateral filaments may be 

 accompanied by generally pronounced constrictions and wall thickening 

 at the sites of branching, and swelling of the intervening regions. These 

 swollen regions form the distinctive utricles. Branching to form the 

 inner utricles is generally tri- or tetrachotomous. 



The branching and rebranching of the lateral filaments is dichoto- 

 mous, and is not accompanied by pronounced constrictions at sites of 

 branching, or by swelling of the intervening regions. The utricles appear 

 like continuations of the branches. 



The second of these patterns occurs in the Opuntia group of species 

 (nodal filaments mostly fusing in pairs for a short distance) (Hillis, 

 1959; Colinvaux, 1968a), as well as in the sections Micronesicae (except 

 for melanesica) and Crypticae (see Table III for species included and 

 later (overleaf) for a discussion of sections). Somewhat modified, it also 

 appears in gracilis, lacrimosa and bikinerisis. 



The first pattern occurs in members of section Rhipsalis and many 

 members of section Halimeda. In members of the Rhipsalis except 

 for favulosa, the diameters of the utricles become progressively larger 

 proceeding towards the medulla. 



(i) Extent of inner cortex. The number of layers of utricles in the 

 cortex may be a helpful taxonomic character for some determinations, 

 although this character is more variable than many of the others. 



The number of layers ranges from two to five, rarely six, with two 

 usual in discoidea, gigas and macrophysa, and five in opuntia and some 

 of the species belonging to section Rhipsalis. Exceptions include the 

 presence of a third layer in some material of discoidea (Subsection (e) 

 above), and sometimes only two layers in lacunalis, where usually 

 there are three. 



