48 TROPICAL PACIFIC ECHINI. 
towards the ambitus. In an adult specimen, with a test 27 mm. in length, 
from Makemo Island (fig. 2), the anal system is elongated, ellipsoid, with pointed 
adoral and aboral ends; and in two other specimens (figs. 5, 6), 27 mm. long, 
from Mauritius, the opening in fig. 5 is broader with the aboral end pointed, 
while in fig. 6 it is more symmetrically ellipsoid. Two young specimens from 
Port Antonio, tests 6 mm. and 10 mm. in length, show the same diversity; 
one (fig. 8), is smaller and an ellipsoid; the other (fig. 9), with its aboral end 
running into a sharp point, has little or no similarity to a specimen from Florida 
(fig. 11), which also differs from an adult individual from Barbados (fig. 12); 
in the latter the opening of the anal system recalls that of the specimen from 
Mauritius (fig. 5). In a specimen 29 mm. long from Bourbon (fig. 7), the out- 
line with its broad rounded aboral end, has no similarity to any of the other 
figures given on Pl. 15. This change in outline and size of the anal system, 
which is probably caused by the amount of resorption in the first six or seven 
pairs of interambulacral plates, makes the anal system valueless as a specific 
character. 
The order in which the plates are arranged on the buccal and anal plastrons 
in other families of the spatangoids has hitherto been regarded as irregular and 
of slight consequence. The figures given on Pl. 15 of the anal plastron show 
a remarkable regularity in the grouping of the plates; about seven to nine of. 
which are laid on the plastron adjoining the interambulacral plates with the 
largest ones in the adoral end slightly raised. The aboral end has three or 
more large plates with the addition of a number of small ones surrounding the 
anal outlet. In the specimens from the Pacific the largest plates on the plastron 
towards the adoral end, are separated at the median line either by a large plate, 
or a double row of rhomboidal ones, while in the West Indian specimens these 
plates are replaced by a number of long slender calcareous strips, which make 
the surface flexible. This is the only differential point I find between the 
Pacific and West Indian specimens, and it accords with Mr. Agassiz’s statement, 
Revision, p. 334-335. 
