64 TROPICAL PACIFIC ECHINI. 
of the inner series in Echinonéus usually appears first just in front of the fourth 
tubercle (from the edge of the actinal system) of the outer row; but in Micro- 
petalon it is in front of the second. The mamelon is without perforation, the 
boss slightly raised, and the scrobicular circle indistinct. Of glassy tubercles 
there are only a few, mostly near the abactinal system. 
Upon comparing the arrangement of the coronal plates in a specimen of 
Echinonéus, 5.90 mm. long, from Jamiaca (PI. 6, fig. 1), with that of Micropeta- 
lon (Pl. 30), I find a notable agreement; and considering the difference in size, 
it is remarkable that there are nearly the same number of plates in the radii of 
both. 
There were probably three sphaeridia in each ambulacrum, in Micropetalon; 
those seen were pear-shaped. Of the pedicellariae I found only the ophicepha- 
lous, and these show little diversity in form and reveal no special characters. 
As in Echinonéus, a few spines near the actinal system have serrations which 
do not continue fully up to the distal end, and the longer spines also vary in 
having nine or more rods. 
One of the notable points is the form of the spicules in the actinal tube-feet; 
there are four disk plates in each, with yoke-shaped spicules at their inner mar- 
gins, while the outer edge of the disk generally has a ring of slender spicules 
slightly bent at both ends. At the ambitus and on the abactinal side near the 
ocular plates the basal disk has diminished in diameter to about 0.11 mm. with 
the four disk plates still less developed. 
What may especially separate Micropetalon from Echinonéus, is the small 
low boss of the primary tubercles, that leaves greater intermediary space; while 
the absence of the thick glossy plates on the oral membrane and the flush 
poriferous zone with the minute ambulacral pores at the ambitus’ suggest the 
retention of juvenile characters. The largest pores are in the primordial ambula- 
cral plates, and one or two of the succeeding plates, and are vertically placed; 
the smallest are arranged diagonally at the ambitus, with a different form of 
peripodium and of pore opening, which are, however, different from those on the 
abactinal side towards the ocular plate where the pores are nearly horizontal. 
The resemblance to Echinonéus and the difference from the adult of that genus 
which the ambulacral pores of Micropetalon show, are well brought out in the 
figures (compare PI. 28, figs. 15-18, Pl. 29, figs. 9-12). 
One specimen of this species was taken during the 1902 cruise of the “‘ Alba- 
tross” at Station 3847. Off Lae-o Ka Laan Light, Molokai, Hawaiian Islands, 
N. 64° 30’, W. 23’. 23-24 fms. bottom of sand and stones. 
These were overlooked when the specimen was first studied (Bulletin M. C. Z., 1907, 60, p. 251). 
