52 TROPICAL PACIFIC ECHINI. 
three are grouped in a triangle with the base upwards. These three first pri- 
maries are recognizable in fig. 3 (8 mm.), and fig. 4 (10 mm.), the latter having 
an increase of two primaries in the lower part, one being towards the outer 
side, the other near the median line. In fig. 5 (12 mm.), the seventh plate is 
not quite normal, but it shows the same regularity. When a length of about 
15 mm. (fig. 6) is reached, the upper row has four primaries, one of the new comers 
being near the outer margin, the other, which is the smallest near the median 
suture. In a test of 17 mm. (fig. 7), and an adult of 26 mm. (fig. 8), there 
is no increase of the primaries, but they are arranged as previously described. 
The miliaries vary in number. The glassy tubercles are minute, few, and fre- 
quently absent. 
While there are only four of the primary tubercles in the upper row in these 
specimens, Pl. 25, figs. 1-8; when the test is broadly ovoid with a length of 
35 mm. or so, they have gradually increased to eight or more. 
A comparison of the primary tubercles on the ambulacrum in specimens 
of various sizes and different localities shows a regular order in their appearance. 
The odd anterior ambulacrum was selected for illustration, and in the extensive 
material examined from Port Antonio, there was no deviation so long as the 
specimens were normal. The smallest specimen, 6 mm. in length (PI. 24, fig. 7), 
has two rows of primary tubercles, one on each side of the ambulacrum adjacent 
to the ambulacral pores. The first primary of the next line, the third, has its 
seat in front of the fourth tubercle from the actinal system as shown, Pl. 24, 
fig. 1-7. This third row in fig. 2, test 8 mm. long, curves towards the left side 
of the ambulacrum, to make room for the fourth which commences on the 
right side near the second tubercle of the third row, as seen in fig. 3, a test of 
9.5 mm., and fig. 4, test 11 mm. long. A fifth row has its first tubercle above 
or close to the third primary of the fourth line, as in two specimens, 16 and 18 
mm. long, figs. 5 and 6. The latter specimen (18 mm.) has the sixth or last 
row to the right of the fifth, beginning at the right of the first tubercle in the 
fifth row. Some of the central rows extend only to the actinal and abactinal 
region of the ambitus. The odd anterior ambulacrum of a test 27 mm. long, 
from the Pacific (fig. 7), shows exactly the same number of rows and order of 
grouping in the primary tubercles as is seen in those from Jamaica. 
The glassy tubercles are mostly ovoid, with their lower half in the test, 
flattened at the top with corrugated sloping sides, armed with small sharp points 
(Pl. 25, figs. 11, 12). They are either colorless, or of a blackish tint, until freed 
from the test when they become transparent. 
