$6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I49 



sand, and some specimens were observed to cover themselves with a 

 layer of sand and carry it with them as they move over the surface 

 without burrowing. This species has the flat ventral surface that 

 characterizes species that burrow. 



The other common Clyp easier in the Coral Reef Preserve is C. 

 rosaceus which we did not find buried, and whose shape is greatly 

 different from that of C. subdepressus. The test is strongly convex 

 dorsally, a feature in common with burrowing spatangoids, but the 

 oral surface is deeply concave, a shape not found in burrowers. The 

 concavity of the oral surface keeps the nearly centrally located mouth 

 raised somewhat above the sand as the animal moves over the surface. 

 This shape would be an impediment to motion in a burrowing species, 

 but probably is advantageous to C. rosaceus in allowing the test to 

 ride over tufts of grass, and then to fill the concave underside as the 

 animal stops to feed. 



Thick or tumid forms such as Meoma ventricosa and Plagiobrissus 

 grandis are somewhat streamlined and have the ventral surface flat. 

 This shape is efficient for burrowing deeper than do the flat sand 

 dollars, and size probably is no impediment to motion, providing the 

 thickness of sand is sufficient. 



All the regular echinoids we saw lived above the surface of the 

 sand. Some such as Echinometra lucunter may live in deep niches in 

 rock. We noted no preferred direction of motion of active regulars 

 such as Lyl echinus variegatus or Tripneustes ventricosus, an observa- 

 tion corroborated by Hyman (1955, p. 550) who reports that L. varie- 

 gatus can walk with any ray forward, and cites Parker (1936) to the 

 effect that the axis of forward motion frequently changes as the ani- 

 mal progresses. 



Sand dwelling regular echinoids live above its surface, and some 

 like Astropyga magnifica (and the presumably nocturnal Diadema 

 antillarum ) arc surprisingly mobile. 



ABNORMAL SPECIMENS 



One tetramerous variant of Meoma ventricosa was found alive at 

 locality 23. This specimen (pi. 16, figs. 5-6) lacks ambulacrum IV 

 (left anterior) and its two associated half-interambulacra. Ocular IV 

 and genital 3 are missing from the apical system. This variant is 

 typical of the tetramerous variant group 4 as described by Jackson 

 (1927, p. 502). Because only four ambulacra are present at the peri- 

 stome (where the oldest, first-produced plates occur), it is apparent 



