50 TROPICAL PACIFIC ECHINI. 
THe SPHAERIDIA. 
Plates 16, 17. 
Thirty-three figures of the sphaeridia from tests of intermediate sizes 
varying from 10 to 34 mm. in length, and from various localities in the West 
Indian and Pacific regions, are given on Pls. 16 and 17. No two are exactly 
alike, neither do they show any specific characteristics; but some of them corre- 
spond in form to those of Maretia or Brissopsis, ¢f. Lovén’s Etudes, Pl. 1. 
An examination of the sphaeridia in Echinonéus shows their height to be 
greater in proportion to the size of the test in the smaller specimens than in those 
full grown. The largest sphaeridium of a young specimen, from Makemo Island, 
10 mm. long, is 0.10 mm. in height; in another from Fakarava Island, 12 mm. 
in length, it is 0.13 mm., and a third from Barbados, with the test 26 mm. in 
length, has the highest 0.20 mm., while an adult specimen from Lord Howe Island 
34 mm. in length or three times as long as the youngest from Makemo Island, 
has its sphaeridia only 0.16 mm. in height. 
Among the material at the Museum I found only three sphaeridia to each 
ambulacrum, although Lovén’s Etudes (Pl. 9, fig. 79), gives four. There are 
searcely two alike from the same ambulacrum; some have an uneven papillate 
surface, but commonly they are almost smooth. The row of sphaeridia (PI. 16, 
figs. 1-4), taken from odd anterior, right anterior, left anterior, and left pos- 
terior ambulacra respectively of a specimen from Fakarava Island, 12 mm. in 
length, vary in form with no indications of specific characters. Of six sphaeridia 
(Pl. 16, figs. 5-8), from two specimens from Lord Howe Island, 30 and 34 mm. 
in length, two (fig. 5) from the odd anterior ambulacrum are 0.16 mm. high, one 
of them being spherical with a wavy outline, the other smooth and pointed at 
the top; one from the left anterior ambulacrum (fig. 6) is depressed with flattened 
top; another from the left posterior ambulacrum (fig. 7), has a spherical outline, 
slightly truncated at the upper end. Two others (fig. 8), from the right anterior 
ambulacrum of a specimen measuring 30 mm. are 0.18 mm. high; one is unevenly 
cylindrical in outline, the other ovoid. This irregularity of form (Pl. 16, figs. 
13, 14), can be traced even in the young test measuring 10 mm. in length. 
In ten figures from the different ambulacra (Pl. 17, figs. 1-6), of two speci- 
mens, from Barbados, measuring 26 and 30 mm. in length, the height of the 
sphaeridia varies from 0.17 mm. to 0.20 mm. This variation which exists 
between the sphaeridia is further accentuated as shown in fig. 1 where the sphae- 
ridium from the right posterior ambulacrum is ovoid with two points at the top, 
its companion being ellipsoid with a sharp upper end. There is still another form, 
