142 L. HLLLIS-COLINVAUX 



Plants erect, moderately compact or somewhat loose, arising from a small 

 holdfast region, to 20 cm tall; calcification relatively heavy; branching 

 somewhat sparse with up to four or five segments and occasionally more 

 arising from a single one; basal segments subcylindrical, subcuneate to 

 subreniform, the three to four lowermost ones often devoid of branches; the 

 otliers brittle, sometimes discoid but more commonly subreniform to 

 reniform with subcuneate segments frequently occurring at the bases of the 

 branches, the outer margin entire or slightly undulating, frequently raised 

 and infolded, measuring to 16 mm long, 25 mm broad, and averaging 

 0-7-1 -0 mm in thickness. 



Cortex of two to three layers of utricles produced by dichotomies in the 

 lateral branches of the medullary filaments ; the outermost utricles remaining 

 slightly attached or separating on decalcification, appearing rounded in 

 surface view, 23-47 fxm in surface diameter, (48-)60-105(-125) [im. long in 

 section, most commonly four but up to eight borne on each secondary 

 utricle; secondary utricles usually not constricted at their origin, 20-47 [i,m 

 broad. 



Nodal medullary filaments occasionally remaining separate but more 

 commonly uniting in twos and threes, the fusions within these units being 

 complete or incomplete; fusion units not particularly entangled, and at most 

 adhering only slightly. 



Type specimen. Marshall Islands, Namu Islands, Bikini Atoll, 

 Taylor 46-156, 3 April, 1946 (MICH). 



Habitat. In deep holes of inner reef flats (Taylor, 1950); it is also 

 known to — 90 m from material dredged during the Sealark 

 Expedition, 1905 (Hillis, 1959). 



Geographic distribution. Western Indian Ocean ; north Pacific. 



In external appearance this species is most commonly confused with 

 specimens of tuna with large segments, with discoidea, and to a lesser 

 extent with gigas. A number of well-defined distinctions exist for 

 bikinensis, however, including the greater degree of calcification of its 

 segments, the dichotomous nature of the inner cortex, the smaller surface 

 diameters of the peripheral utricles, and the relative ease of separation 

 of these utricles. This last characteristic varies somewhat with age. 



Microscopically, this species appears to have most in common with 

 gracilis and lacrimosa. Characteristic of these two species and most 

 particularly lacrimosa are the somewhat swollen apical ends of the 

 secondary utricles which in turn support several, usually 6-18, peripheral 

 utricles. These are absent in bikinensis, although the appearance is 

 somewhat simulated by some of the broader secondary utricles which 

 may bear up to eight peripheral utricles rather than the more usual 

 two or four. 



