THE GENUS COLOBOCENTROTUS. 15 
The actinal system of Podophora atrata is pentagonal (Pls. 20, fig. 2; 
21, fig. 1). The lips of the gill cuts are short, sharply marked; the proportion 
of the actinostome to the test is as 23 to 38. There are five pairs of small 
rectangular poriferous plates. The buccal membrane is only covered by a few 
round or elliptical plates irregularly scattered and with five clusters of narrow 
crescent-shaped plates placed distally next to the pairs of poriferous plates. 
In a smaller specimen (PI. 25, fig. 2) the poriferous buccal plates form a 
nearly closed ring, the elongated buccal plates are less closely packed than in 
older specimens, and the isolated circular plates are more distant. 
The changes which take place in the interambulacral zones during the 
growth of the test are shown on Pl. 24, figs. 1-4, giving the left anterior 
interambulacrum of specimens measuring 8, 14, 26, and 45 mm. in diameter. 
In fig. 1, the same as PI. 25, figs. 1-5, there are six and seven interambulacral 
plates, each carrying a large or small primary tubercle, with a number of 
miliaries or small secondaries along the median line and the outer edge of 
the plates. 
On Pl. 24, fig. 2, a specimen measuring 14 mm. in diameter, there are 
eight and nine interambulacral plates, and a second vertical row of primary 
tubercles is forming along the median line of the fourth and fifth plates, as 
well as another vertical row of small secondaries on the outer edges of the 
third, fourth, fifth, sixth, and the second, third, fourth, fifth plates counting 
from the actinostome. 
On Pl. 24, fig. 1, these two outer vertical and median rows can be detected 
by the greater size of the small secondary on each side of the two primary 
rows in the position of the new vertical rows well developed in fig. 2. In 
the next stage of the same interambulacrum figured, a specimen measuring 
26 mm. in diameter (Pl. 24, fig. 3) with eleven and eleven plates, the four 
vertical rows of primary tubercles are well developed. The first rows, so 
prominent in figs. 1, 2, are still readily distinguished from the two inner 
and the two outer rows by the greater size of the primary tubercles, which 
extend from the second to the seventh plate, while the two inner rows extend 
the one from the fourth to the eighth plate, the other from the fifth to the 
eighth plate. 
In the specimen, figured on Pl. 24, fig. 2, the miliaries are proportion- 
ally much more numerous than in the younger specimen (PI. 24, fig. 1); they 
are quite crowded in the next stage (PI. 24, fig. 3), and in the older specimen, 
45 mm. in diameter, they are somewhat more openly arranged (PI. 24, fig. 4). 
