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 neai'ly 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 (PI. 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 PI. 25, 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 PI. 24, fig. 1, and PI. 25, 

 figs. 3, 4. PL 25, fig. 1, shows the pavement of i-adioles 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 PI. 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. In a specimen of 26 mm. the small radioles of 

 two of the plates of the outer rows can be distinguished; those of the inner 

 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 PL 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. 



