ECOLOGY AND TAXONOMY OF Holimeda 95 



immediately above the first to second basal segments, the resultant segments 

 at times becoming consolidated laterally into a rather massive fan-shaped 

 structure; the basal first to second segments cylindrical to subcuneate, 

 others plane, ribbed or keeled, shape variable, from cylindrical to reniform, 

 the outer margin entire, undulating or deeply lobed, to about 10 mm long, 

 14 mm broad, and averaging 0-75-1 -00 mm in thickness. 



Cortex of three to five layers of utricles ; the outermost utricles generally 

 remaining attached after decalcification for an average distance of 12 \xxa., 

 their lateral and peripheral walls occasionally thickened, (34-)45-85(-105) (xm 

 in surface diameter, (40-)50-95(-125) (jim long in section, two or four 

 supported on each secondary utricle; secondary utricles often globose or 

 subglobose, 23-70(-95) [xm broad, 30-90(-118) \xm long; innermost utricles 

 (32-)46-100(-150) pim broad. 



Nodal medullary filaments uniting as a single group for a distance of 

 approximately 25-80 jxm, the adjacent filaments communicating by pores or 

 tubular processes ; walls in this region thickened and often deeply pigmented. 



Type specimen. Collected by Ellis in Jamaica; according to 

 Barton (1901) this specimen has been lost. 



Habitat. Grows in sand, mud or other unconsolidated substrate 

 from just below low-tide line to about — 65 m. In the Caribbean it 

 may form very dense stands, often with H. monile, of more than 

 300 thalli of the two species per square metre just below low-tide 

 line. In shallow environments that are somewhat exposed, as 

 opposed to sheltered, the long axis of older plants is perpendicular 

 to the direction of the wave motion. 



Halimeda incrassata frequently is associated with sea grasses. In 

 the Caribbean H. monile and H. simulans may occur with it. It is also 

 associated with species of Penicillus, JJdotea and Bhipocephalus. 



At Enewetak Atoll it was found in moderately exposed to sheltered 

 sites and in more exposed habitats than cylindracea. Extensive patches 

 of it such as are common in the Caribbean were not observed. 



Geographic distribution. Pantropical; includes western and 

 eastern Indian Ocean; north and south in the western Pacific; 

 north and south in the western Atlantic including Bermuda 

 (western Atlantic). 



This species is best known from the Caribbean where it appears 

 to be the commonest of the Rhipsalian taxa, and often forms 

 extensive populations in the shallow sandy regions of the reef. 



Modifications other than those of segment shape occur in specimens 

 of incrassata. These include an increase in the number or height of the 

 cortical layers, either or both conditions producing thicker segments. 



