60 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I49 



Species whose distribution is controlled by the distribution of turtle 

 grass might be said to depend on substrate, but as depth controls the 

 turtle grass, it also indirectly controls the echinoids. 



Species controlled by distance from shore. — Meoma ventricosa 

 inhabits waters of greatly varying depth, but was not encountered 

 nearer than about 4 miles from shore, just a short distance landward 

 from the reef. Although it was found in shallow water on White 

 Bank, it was not found in deeper water nearer shore. 



Species not evaluated. — Factors controlling the distribution of 

 Diadema antillarum could not be evaluated within the area studied. 

 The species was found in all depths, on all substrates and its range 

 extends from shore to the seaward limit of the area. 



The habitat of Echinometra viridis is not well enough known to 

 evaluate factors controlling its distribution. At present it seems to 

 inhabit only niches in rock, and the shadows of sponge and coral 

 heads, in the manner of Arbacia punctulata. Furthermore, its depth 

 tolerances are unknown. 



Echinoid associations. — Certain echinoid species characteristically 

 occur near one another as inhabitants of similar or contiguous en- 

 vironments. Five such associations were encountered in the area 

 studied. 



1. Echinometra lucunter and E. viridis live in holes in rock near 

 shore, and under the shadows of corals and sponges in shallow water 

 just offshore. 



2. Lytechinus variegatus, Tripneustes ventricosus, Clypeaster 

 rosaccus and Ettcidaris tribuloidcs live above the surface of the sand 

 in turtle grass. In addition, Arbacia punctulata and Eucidaris tribu- 

 loidcs cluster around isolated mounds of coral or sponge within these 

 grassy areas. Diadema antillarum also inhabits the turtle grass, with 

 adults assembled into groups, and juveniles living singly. 



3. Clypeaster subdepressus, Encope michelini and Leodia sexies- 

 perforata inhabit the upper layer of clean grassless sand, and fre- 

 quently are found together. Meoma ventricosa and Plagiobrissus 

 grandis inhabit the same areas, although they burrow more deeply. 

 Clypeaster rosaceus also may be encountered on the same grassless 

 sand, but normally only where grassy patches occur in the vicinity. 



4. Diadema antillarum appeared to be nearly the sole inhabitant 

 of niches within the main body of the reef, although one specimen of 

 Echinometra viridis was found in a hole in the reef. 



5. Brissus unicolor and Echinoneus cyclostomus live under de- 

 tached rocks in sandy patches within the reef area. 



