284 A. E. Verrill—The Bermuda Islands; Coral Reefs. 
This is the most common species at Bermuda as well as in the 
West Indies. It forms broad, coriaceous, pale yellow crusts, due to 
the thick coat of caleareous sand, on the ledges and reefs, both at 
low tide and in deeper water. These colonies are often two to four 
feet across and from one-third to one-half an inch (S-12™™") or more 
in thickness when living, and with a nearly smooth surface when the 
polyps are entirely retracted. But when they expand they can rise 
considerably above the ccenenchyma, the projecting portion being 3 
129a 
129 
Figure 129.—Palythoa mammillosa, part of a colony, with some of the polyps 
expanded ; nat. size. 
Figure 129a.—The same. Another part of the same colony, nat. size. Both 
phot. while living, under water, by A. H. V. 
to 5™" high, cylindrical, or expanding toward the disk, conical, or 
hemispherical according to the degree of expansion (figs. 129, 130). 
When dried, therefore, these crusts sometimes show slight mam- 
milliform elevations over the polyps, and sometimes depressions, or 
even round openings (as in ocellata of Ellis and Sol.). 
The tentacles are small and short, about .05™" long, varying from 
34 to 40, but usually 36 to 38 in full grown polyps. They form two 
alternating rows, with an outer circle of whitish, angular, columnal 
lobes, opposite the inner circle of tentacles and of the same number. 
