THE GENUS COLOBOCENTROTUS. 23 
plate only six; the ambulacral plates are much cut up, there being seven plates 
and demi-plates in the length of two plates. 
The poriferous arcs extend down toa little above the ambitus (Pls. 8, 
figs. 2, 3; 9, figs. 2, 3), where the regular arrangement of the pores begins 
to be disturbed and they become crowded laterally, see Pls. 8, fig. 1; 9, 
figs. 2, 3. Below the ambitus on the actinal side the pairs of pores are thrown 
aside laterally and greatly crowded (Pl. 13, figs. 3, 4), as is shown in Pl. 13, 
fig. 3, on the actinal side, and Pl. 13, fig. 4, from the interior of the same 
plates. There are but few miliaries in the poriferous field (Pl. 13, fig. 3). The 
passage of the regular arcs into the broad poriferous field is well shown for 
P. atrata on Pl. 25, figs. 6-10. The apical system and the arrangement of 
the pores of the abactinal part of the ambulacral system of a large speci- 
men measuring 82 mm. is seen on PI. 14, fig. 4. In the second plate from the 
apex the upper demi-plate is followed by a row of seven poriferous plates which 
extend across the plate towards the median line, and beyond the projection 
of the large plate and its last pair of pores. When we come to the fifth 
ambulacral plate, counting from the actinal system, there are eleven pairs of 
pores arranged in a huge arc round the base of the primary tubercle. The 
upper pair is pushed out laterally well toward the median line (PI. 13, fig. 6). 
The corresponding ambulacral plates seen from the interior (Pl. 13, fig. 7) are 
seen to be made up of a large plate and a narrow, elongated, and smaller plate, 
the large plate having nine intercalated poriferous demi-plates extending about 
two thirds across it from the side of the interambulacrum; the small plate 
having two intercalated poriferous plates. The inner line of sutures of the 
intercalated poriferous plates forms a slightly zigzag-shaped vertical line 
more or less parallel to the median ambulacral line. 
In P. pedifera the actinal system is proportionally larger (Pls. 8, 
fig. 1; 9, fig. 2) than in P. atrata. The ratio of the actinostome to the test 
is as 28 to 46. The pairs of poriferous actinal plates are elliptical and larger 
than in P. atrata; they are more distant from the teeth and are fairly covered 
with secondaries and miliaries. The buccal membrane is nearly covered with a 
belt of crescent-shaped plates somewhat more closely packed distally from 
the five pairs of poriferous plates (Pl. 13, fig. 2). Between the teeth and 
the buccal poriferous plates there is a ring of small elliptical plates (Pls. 7, 
fig. 1; 8, fig. 1; 9, figs. 1, 4; 13, fig. 2). 
The thin folds of the actinal cuts extend almost to the ambitus, enclos- 
ing in a sort of furrow the two rows of small secondary interambulacral 
