152 L. HILLIS-COLINVATJX 



keeled, occasionally cylindrical, these usually occurring as the basal segments 

 of branches, sometimes subcuneate but more commonly subreniform to 

 reniform, the lower margin truncate to auriculate, the upper margin entire 

 to trilobed, to 9 mm long, 16 mm broad, and averaging 0-50-0-75 mm in 

 thickness. 



Cortex of three to four layers of utricles produced by successive 

 dichotomies in the lateral branches of the medullary filaments; outermost 

 utricles separating on decalcification, appearing rounded in surface view and 

 at tunes thickened along the peripheral margin, 21-52 (j,m in surface 

 diameter, (32-)43-81 [xm long in section, borne two or four on each secondary 

 utricle; secondary utricles usually not constricted at their origin; 15^6 fjim 

 broad. 



Nodal medullary filaments remaining unfused although adjacent filaments 

 may adhere slightly ; filament walls in this region usually extremely thickened 

 and deeply pigmented. 



Type specimen. Marshall Islands, Enewetak Atoll, Taylor 46-394, 

 2 June, 1946 (MICH). 



Habitat. Growing on rock in the fast-moving waters of inter- 

 island passes at Enewetak where it seemed to be rare; reported 

 commoner in the deeper waters of the lagoon attached to pinnacles 

 (Gilmartin, 1960). The known vertical range of the species of 

 — 1'5 m to —57 m. 



Geographic distribution. Central Indian Ocean; north-western 

 Pacific. 



Macroscopically, fragilis is most frequently confused with opuntia, 

 gracilis and to a lesser extent bikinensis. In all four species the cortex 

 is formed essentially by dichotomous branching of the lateral medullary 

 filaments and thus internally there are also similarities. Difficulties in 

 macroscopic determination, however, can always be resolved by an 

 examination of the nodal medullary filaments which remain separate 

 in fragilis, but fuse in twos and threes in the other species. The separate 

 peripheral utricles in fragilis are also distinctive. In bikinensis the 

 peripheral utricles separate only with pressure, whereas they generally 

 remain attached in opuntia and gracilis. 



From micronesica, a second species in which the nodal filaments 

 remain separate, fragilis can be distinguished macroscopically by its 

 heavier calcification and by its often complanate branching. In addition, 

 the segments are somewhat thicker, larger, more brittle and commonly 

 reniform in shape rather than discoid or trilobed, and the lowermost 

 segment is usually smaller than the others rather than being generally 

 much larger and flabellate as is tjrpical for micronesica. 



