RECORDS OF IV. A. MUSEUM. [i59 
43 mm. wide ; ambulacra nearly 13 mm. ; hence interambulacra 
more than three times as wide as ambulacra. But median interam- 
bulacral space only 11 mm. wide, and hence only one-fourth of 
interambulacrum. Median ambulacral area less than 6 mm. wide, 
and hence less than half the ambulacrum. Abactinal system 28 
mm. in diameter and actinostome the same. 
Genital plates moderately large, about n mm. wide by 7 mm. 
high, but madreporic genital very large, nearly 16 mm. wide by 11 
mm. high. Oculars small and low ; ocular V insert, and I nearly so ; 
in the smaller specimen, all the oculars are exsert but V is nearly in. 
Genital pores large, near centre of plate, surrounded by a circle of 
about a dozen broad flat spinelets larger than the others on the 
plate ; as these spines are closed over the pores, they form a con- 
spicuous conical elevation. Ocular pores small near distal margin 
of plate. Interambulacra, median areas of - ambulacra, abactinal 
system and actinostome, densely covered with small flattened 
bluntly pointed spinelets and pedicellariae. Secondary spines of 
interambulacra in circles of 15-20 around the bases of the primary 
spines ; they are broad, flat and truncate, about 8 mm. long by 2 
mm. broad. Along the margin of the median area of each ambula- 
crum is a series of narrow, flat spines, about 4 mm. long and .5 mm. 
wide, which lie flat across the poriferous areas. Between these 
marginal series are four or five series of smaller spines or spinelets 
similar to those on the interambulacra. Pores of a pair rather large 
wider than high, separated .by a space wider than the width of a 
pore and connected by the groove characteristic of the genus. 
Pedicellariae present in great numbers. The tndentate show 
little diversity in size, but are chiefly actinal in position and on the 
interambulacra. Their valves are about two millimetres long, but 
the blade is only about .25 mm. wide, so they are very slender ; the 
valves meet only at or near the tip. There is a single vertical ridge 
extending the length of the blade in the median line on its inner 
surface similar to those found in the tridentate pedicellariae of P. 
imperialis, but rather more prominent ; it is sharply serrate, more 
coarsely so than the margins of the blade. The small globiferous 
pedicellariae are abundant almost everywhere and are very similar 
to those figures by Mortensen for P. Imperialis. Their valves are 
about .30 mm. long. The large globiferous pedicellariae are very 
common, particularly on the ambulacra ; the valves are .80-. 90 mm. 
