24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I49 



though the practice also produces a camouflage that appears to be 

 effective and probably necessary during the daylight hours (for 

 further discussion, see Clypeaster rosaceus). 



Order Temnopleuroida Mortensen 



Family TOXOPNEUSTIDAE Troschel 



Genus Tripneustes Agassiz 



TRIPNEUSTES VENTRICOSUS (Lamarck) 



Plate 3, figure 2 ; text figure 4 



The test of this urchin is large — an adult approximately 110 mm. 

 in horizontal diameter. It is brown and has numerous short, white 

 spines. The bottom of the test is relatively flat, the top domed with 

 smoothly curving sides. The bare test is characterized by broad ambu- 

 lacra with six vertical rows of pore-pairs in each area. The inter- 

 ambulacra are slightly wider than the ambulacra and have many small 

 tubercles arranged in horizontal rows. This species resembles 

 Lytechinus variegatus with which it is associated and often confused, 

 but differs in its generally larger test, persistent brown color in living 

 specimens, less conspicuous pedicellariae, and more numerous and 

 shorter spines. Its bare test is distinguished by its white color, and its 

 more widely separated pore-pairs in each poriferous zone. 



Occurrence. — Tripneustes ventricosus was found in grassy areas on 

 sand bottoms. Its habitat is similar to that of Lytechinus variegatus, 

 but seems to be more restricted in this area. Where the two species 

 occur together, T. ventricosus is much less abundant. L. variegatus 

 occurred at nearly all stations where turtle grass was abundant, but 

 T. ventricosus was found only in the offshore grassy areas, on the 

 White Bank, in the "back-reef channel" and in the "interreef deep 

 channel" (figs. 3, 4). Depth does not appear to be the controlling 

 factor, because the species was found in waters from 5 to 35 feet 

 deep, a range very similar to that of L. variegatus. 



Immature individuals about an inch in diameter were found on 

 open sand near an extensive patch of grass at station 30, clustered 

 together with immature L. variegatus in clumps of algae, broken 

 shells, and small sponges and sponge fragments. Similar juveniles 

 were found under rocks in only 5 feet of water at station 38, in ab- 

 sence of L. variegatus. Lewis (1958, p. 607) found mature specimens 

 as well as immature ones on rocky bottoms off the coast of Barbados, 

 B.W.I. Lytechinus variegatus was absent from this habitat in the 

 study area and has not been reported with certainty from Barbados. 



