16 TROPICAL PACIFIC ECHINI. 
In this specimen there are twelve and twelve plates; the outer vertical rows 
extend from the second to the ninth and perhaps to the tenth plate, the inner 
median rows from the fourth to the ninth plate. There still exists a marked 
difference in the size of the primaries of the first vertical rows near and at the 
ambitus compared to the primary tubercles of the other rows, which, with the 
exception of the upper tubercles, are nearly of the same size. It will be 
noticed that the outline of the scrobicular areas and of the mammary boss 
varies greatly; they are circular, angular, or elliptical according to their posi- 
tion in the plate and the amount of space available (see Pl. 24, figs. 5, 
7, 8). These changes are due to the great pressure brought about, especially 
at the ambitus, by the sudden change of direction in the plates passing from 
the abactinal side to the ambitus and to the actinal surface. It will be seen 
that in the large specimen (PI. 24, fig. 4) there are on the actinal side four 
vertical rows of small secondary tubercles separating the adjoining interam- 
bulacral rows (Pl. 24, fig. 4). 
The order of appearance of the tubercles of the vertical rows can be 
equally well traced in specimens covered with radioles. On Pl. 2, fig. 1, is 
seen a young specimen 14 mm. in diameter with the spines having but 
two vertical rows of large primary tubercles, as in Pl. 24, fig. 1, andPl. 25, 
figs. 3, 4. Pl. 25, fig. 1, shows the pavement of radioles of the five ambula- 
cral and interambulacral zones, each with two rows of primary radioles and 
the pavement covering the abactinal system. This is shown more in de- 
tail on Pl. 22, fig. 1. In fig. 2 of the same plate, in a somewhat larger 
individual, we can see the first trace of the spines of the inner vertical row 
of tubercles. This is somewhat more advanced in the next largest speci- 
men figured, Pl. 22, fig. 3. Ina specimen of 26 mm. the small radioles of 
two of the plates of the outer rows can be distinguished; those of the imner 
median rows are well seen. In the next stage these radioles have all become 
larger and more prominent (Pl. 22, fig. 4). In the following stages, from 
specimens measuring 31 mm. to 77 mm. in diameter, the mode of appearance 
of the radioles becomes more and more apparent and the vertical lines of 
primary tubercles more distinct (Pl. 22, figs. 5-10). In Pl. 22, figs. 5-7, the 
distinctness of the outer vertical zones and of the inner median zones becomes 
clearer with increasing size, and in the specimens of PI. 22, figs. 8-10, of 67 to 
77 mm. in diameter, the dotted lines on the radioles show the course of the 
primary vertical row with the dotted lines marking the position of the lateral 
and median vertical lines. 
