NO. 6 ECIIINOID DISTRIBUTION AND HABITS — KIER, GRANT 47 



Summary. — The following changes occur during the growth from 

 22 mm. in length to full size : 



1. Test becomes lower, more angular, petals become depressed, 

 posterior truncation tilts so that periproct is visible adorally. 



2. Posterior petals become much longer, all petals become rela- 

 tively narrower and curved at the ends. 



3. The peristome becomes smaller relative to the length of the 

 test, and the labrum enlarges and extends anteriorly. 



4. The spines become more equal in size, and proportionately 

 much shorter. 



5. The subanal fasciole becomes discontinuous because of the 

 elimination of that portion of the fasciole nearest the periproct. 



6. Genital pores are introduced when a specimen is approxi- 

 mately 40-60 mm. long. 



Most of these growth changes also occur in Plagiobrissus grandis 

 (Gmelin). 



Occurrence. — Meoma ventricosa occurs in sandy areas relatively 

 far from shore. It is most abundant, and reaches its maximum size, in 

 areas of clean sand that are free or nearly free of grass or dense 

 patches of algae. However, specimens somewhat below the normal 

 size were found in turtle grass, along with Clypeaster rosaceus. Their 

 range extends from the inshore edge of the White Bank to the sandy 

 terraces offshore from the main reef (and possibly also beyond, out- 

 side our area of study) . It is most abundant on the White Bank and 

 in the "interreef deep channel," and rare on the deep terraces beyond 

 the reef. The inshore limit of its range overlaps only slightly with the 

 outerlimit of Lyt echinus variegatus; the two are seldom found to- 

 gether (fig. 5). 



The depth range of M. ventricosus is between 10 and 85 feet ; speci- 

 mens were not found at 110 feet, but a wider search at that depth 

 might disclose them. The species is most abundant in depths between 

 20 and 40 feet, and in some areas within that depth range a specimen 

 can be dug up about every two or three feet of traverse. An area 40 

 feet by 40 feet at station 19, in 40 feet of water produced 40 speci- 

 mens of M. ventricosa in about 20 minutes of searching, during which 

 time only 6 specimens of Clypeaster subdepressus were discovered in 

 the same area. 



This large and thick species requires rather deep sand for its bur- 

 rowing, so it was absent from small patches of sand within rocky or 

 reefy areas where a few C. subdepressus could survive. 



