ECHINONEUS CYCLOSTOMUS. 59 
form and large perforations illustrate the variation that obtains in the same 
specimen. 
The open rectangular shape of the first pore in the edge of the actinostome, 
in a specimen 37 mm. long, from Mauritius, while not of common occurrence, is 
far from exceptional in specimens from other localities. 
Any peripodium would have sufficed to show the variation in shape, but for 
uniformity the fifth one (PI. 28, figs. 16, 20, 22-24), in the odd anterior ambula- 
crum of five adult specimens from the Indian, Pacific, and West Indian regions 
was selected. 
THE SPINES. 
Plate 26, figs. 1-15, 20-29; Plate 27, figs. 1-11, 14-22, 25-34, 37-42. 
There are only two kinds of spines; one on the primary tubercles, the other 
on the miliaries (Pl. 26, figs. 1-15, 20-29; Pl. 27, figs. 1-11, 14-22, 25-34, 37-42). 
Those of the primaries are without serrations, some of them on the actinal side, 
especially those near the oral and anal systems in adult specimens, reach a length 
of three millimetres or so. The radioles on the abactinal side become stouter 
with either a blunt or pointed end, and gradually diminish in length until in the 
apical system they measure less than half a millimetre, and on the madreporic 
plate not quite one fourth of one. 
I have been unable to find any difference between the radioles of specimens 
from the Pacific or the West Indian regions, excepting a slight individual 
variation. The base on the longer spines agrees in being higher at one side and 
thus giving the milled ring a sloping line when seen in profile. The sections 
of these spines given on Pls. 26, 27, show either about nine or twelve rods in each; 
exceptionally one or more of these rods (as seen in Pl. 26, figs. 2, 3; Pl. 27, 
figs. 8, 9, 30, 38, 40), is serrated near the base, or for the whole length. The 
various forms in the outline of the sectional figures are due to the shape of the 
rods, which in some spines are semicylindrical, while in others they are much 
more prominent and deeply separated from each other. 
Radioles of the same length, and from the same plate may differ noticeably 
from each other according as to whether they have a pointed or a rounded tip. 
This is shown by two spines (PI. 26, figs. 6, 7) from the ambitus, left anterior inter- 
ambulacrum, of a Pacific specimen, 34 mm. long. A similar difference is also 
easily seen by comparing two spines from a Barbados specimen, 30 mm. long; 
one (PI. 27, fig. 18) from the ambitus, right anterior interambulacrum; the other 
(Pl. 27, fig. 17) from the odd posterior interambulacrum near the anal system. 
