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 to a little above the ambitus (Pis. 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 Pis. 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 (PI. 1.3, figs. 3, 4), as is shown in PI. 13, 

 fig. 3, on the actinal side, and PI. 13, fig. 4, from the interior of the same 

 plates. There are but few miliaries in the poriferous field (PI. 13, fig. 3). The 

 passage of the regular arcs into the broad poriferous field is well shown for 

 P. atrata on PI. 25, figs. 6^10. The apical system and the arrangement of j"7, 9, 

 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 (PI. 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. indifera the actinal system is proportionally larger (Pis. 8, 

 fig. 1; 9, fig. J) 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 (PI. 13, fig. 2). Between the teeth and 

 the buccal poriferous plates there is a ring of small elliptical plates (Pis. 7, 

 fig. i;8, fig. i; 9, figs. 1,4; 13, fig. ^). 



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 



