10 TROPICAL PACIFIC ECHINI. 
interior of the test (Pl. 42, fig. 8, and Pl. 43, fig. 3), it is easy to see the struc- 
ture of the upper ambulacral plates. Each plate is made up of a narrow plate 
reaching from the median line to the outer edge of the plate and of a 
larger plate into which run from the outer edge half way to the median line 
seven demi-plates carrying seven pairs of pores. In the tenth plate from the 
actinostome in which the eight pairs of pores are only slightly crowded to one 
side (Pl. 42, fig. 5), we see the arrangement of the three large primary 
tubercles and the intertubercular space occupied by the miliaries and small 
secondaries. A view from the inside of the left posterior ambulacrum shows 
the sutures indicating the composite structure of the ambulacral plate 
(Pl. 42, fig. 6). In the narrow upper plate the pores marked with a small x 
correspond to the upper pair of pores of Pl. 42, fig. 5, also marked with an x 
and on the same figure we can trace the suture of the demi-plates of the seven 
other pairs of pores forming the poriferous arc which extend half way from 
the outer edge towards the median ambulacral sutures. 
In an exterior view from the actinal side of the poriferous field of the odd 
anterior ambulacrum (Pls. 34, fig. 1; 38, fig. 7; 42, fig. 3) it is impossible 
to detect either the sutures of the primary plates or of the demi-plates ; but in 
a view from the interior (Pl. 42, fig. 4) the peculiar shape assumed by the 
primary actinal ambulacral plates and demi-plates, owing to the crowding and 
compressing of the poriferous plates is shown in Plate 42, fig. 4. The 
pores marked with x in fig. 4 in the two ambulacral plates correspond to the 
pairs of pores similarly marked on Pl. 42, fig. 3. The poriferous field is 
marked by the number of miliaries and small secondary tubercles interspersed 
between the pairs of pores. (Pl. 42, fig. 3.) 
The outline of one of the pairs of poriferous buccal plates of Colob. Stimp- 
soni is shown on Pl. 42, fig. 2. They each carry a few miliaries and a num- 
ber of small secondaries. The plates are originally elliptical; they abut in 
older specimens. 
Owing to the thickness of the test the passage for the tubes leading to 
the pedicels slants very considerably and distant pairs as seen from the inte- 
rior on figs. 4, 6, Pl. 42, are thus brought closely together on the exterior side 
of the test figs. 3, 5, Pl. 42. 
In figs. 3, 4, 5, 6, of Pl. 42, the two minute crosses indicate the position 
of the pairs of pores on the inner and outer faces of the test; figs. 3, 4 on the 
actinal part of the poriferous field of the odd anterior ambulacrum and figs. 
5, 6 of a pair of pores of the abactinal part of the odd anterior ambulacrum of 
