6 TROPICAL PACIFIC ECHINI. 



with a few miliaries on each side of the median line separating the two porif- 

 erous fields. The difference in appearance of the pores of the actinal edge of 

 the poriferous field of the region at the ambitus and those of the abactinal side 

 of the ambulacral system is shown on PI. 46, figs. 2, 3, J,.. 



The odd anterior ambulacrum of a specimen of Podophora atrata meas- 

 uring 46 mm. in diameter is at once distinguished from that of Podophora 

 pedifera by the more regular outline of the vertical rows of primary tuber- 

 cles. Both the mammary boss and scrobicular area are circular in outline 

 (PI. 46, fig. 5) ; they occupy a smaller arc of the ambulacral plates, having a 

 larger space free from miliaries along the median line of the ambulacral zone. 

 There are here and there a few small secondary tubercles in the inner angles 

 of the ambulacral plates. 



The vertical rows of primary tubercles extend as small tubercles to the 

 edge of the actinal system, and numerous minute secondaries and miliaries are 

 intercalated between the pairs of pores of the poriferous field. This is compar- 

 atively narrower than that of Podophora jicdifera, leaving a much broader 

 actinal interambulacral zone to separate the poriferous fields of adjoining 

 ambulacra (PI. 24, fig. J) than in Podophora pedifera. The change in the 

 pairs of pores as we pass from the actinal edge to the ambitus and to one 

 of the abactinal pairs of pores are seen in PI. 46, figs. 6, 7, 8. 



In a specimen of Colobocentrotns Stimpsoni measuring 65 mm. in longest 

 diameter the primary tubercles above the ambitus are of nearly the same 

 size with a circular mammary boss and scrobicular area (PI. 46, fig. 9), those 

 at the ambitus being slightly larger and those on the actinal side of the ambi- 

 tus decidedly larger. In these the mammary boss and the scrobicular area are 

 somewhat elliptical. The abactinal primary tubercles form above the ambitus 

 four slightly undulating vertical rows with small miliaries on both sides of 

 the median line and miliaries between the outer and inner vertical rows. 

 Below the ambitus there are four irregularly placed large primary tubercles 

 with a somewhat smaller one, and below that two vertical rows of small, dis- 

 tant secondaries on each side of the median line extending to the actinal 

 system, with numerous miliaries and minute secondaries filling the spaces 

 between the pairs of pores of the poriferous field. 



On the abactinal side of the test there are usually three large tubercles on 

 each plate ; near the abactinal system they are reduced to two or to one with 

 a small secondary. The poriferous arcs occupy the outer part of the 

 ambulacral plates. There are eight pairs of pores arranged in a very open 



