156 L. HILLIS-COLINVAUX 



generally retaining their hexagonal appearance in surface view, (36-)56- 

 76(-103) (j,m in surface diameter, (42-)54-69(-83) y.in long in section includ- 

 ing the sometimes prolonged pedicels, usually four supported by each 

 secondary utricle; secondary utricles (20-)27-40(-65) [xm broad. 



Plants uniaxial, the single medullary filament generally broadest in the 

 node region where the wall is often yellowish and (ll-)18-45(-58) [xm in 

 maximum thickness. 



Type specimen. Jamaica, Parish of St Ann, Runaway Bay off 

 Cardiff Hall beach, from a depth of 34-41 m, T. E. Goreau and 

 E. A. Graham, 26 August, 1962 (IJ; isotypes in BM, NY, UC, 

 UCWI, UM). 



Habitat. Hanging from rock surfaces or flattened against them, 

 on the fore-reef, fore-reef slope and deep fore-reef, this species 

 develops its largest populations in the deep fore-reef (Goreau and 

 Goreau, 1973; Moore et al., 1976). Down to — 100 m it is associated 

 with copiosa, but cryptica is much the commoner. Its vertical range 

 is —25 m to —100 m. 



Geographic distribution. Western Atlantic Ocean. 



The external aspect of these plants suggests the Indo-Pacific species 

 fragilis. Yet cryptica is readily separated from this and all other described 

 species of Halimeda, with the possible exception of some specimens 

 of copiosa, by the consistently different colour of its lighted and 

 umbral surfaces, the light-exposed surfaces usually being green, the 

 umbral ones white or off-white. Less striking but equally characteristic 

 of cryptica is the especial delicacy of the nodes, an expression of this 

 species' uniaxial construction (Figs 3, 15). All other described 

 Halimedae are multiaxial (Hillis, 1959). Smallish utricles from the 

 upper, and to a lesser extent from the lower, of two adjoining segments 

 form a collar (Fig. 3) which almost covers the medullary filament at the 

 node, and produces the beaked or stalked macroscopic appearance of 

 the upper of the segments. 



The lighted and umbral surfaces of mature segments can be readily 

 distinguished microscopically also. The peripheral cortical utricles of 

 the umbral surface generally seem delicate. They collapse easily, and 

 seem to become detached readily from the secondary utricles. To 

 observe them satisfactorily the mature segments must usually be very 

 slowly decalcified. Preparations to show the surface of the peripheral 

 utricles of the umbral surface are more easily made from immature 

 segments. In contrast, the peripheral utricles of the lighted sm^face and 

 of the edge of the segment continuing slightly into the umbral surface 



