10 TROPICAL PACIFIC ECHINI. 



interior of the test (PL 42, fig. 8, and PI. 43, fig. 3), it is easy to see the struc- 

 ture of the upper arabulacral plates. Each plate is made up of a narrow plate 

 reacliing 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 (PI. 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 4 hc left }303tcrior ambulacrum shows "^ JU_ 

 the sutures indicating the composite structure of the ambulacra! plate ' 



(PI. 42, fig. 6). In the narrow upper plate the pores marked with a small x 

 correspond to the upper pair of pores of PI. 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 (Pis. 34, fig. 1; 38, fig. 1; 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 (PI. 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- 

 soiii 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. Jf, 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 



