THE GENUS COLOBOCENTEOTUS. 15 



The actinal system of PodojjJiora atrata is pentagonal (Pis. 20, fig. Z; 

 21, fig. l). The lips of the gill cuts are short, sharply marked; the proportion 

 of the actinostome to the test is as 23 to SS. 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 PI. 24, figs. l-J^, 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 primai'y tubercle, with a number of 

 miliaries or small secondaries along the median line and the outer edge of 

 the plates. 



On PI. 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 PI. 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 (PI. 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 PI. 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. 5), and in the older specimen, 

 45 mm. in diameter, they are somewhat more openly arranged (PI. 24, fig. 4). 



