02 PANAMIC DEEP SEA ECHINI. 



In the specimen of 53 mm. in diameter the position of the original 

 vertical row of primary tubercles is well shown (PI. 4U, fig. 3). The actinal 

 system measured 10 mm. in diameter, with five rows of buccal plates cov- 

 ering the whole actinal system, and leaving but a slight bare interambula- 

 cral area between the buccal plates; these have become greatly elongate, 

 compare PI. 42, fig. / and PI. 48. fig. 6. 



The ambulacra] area of this specimen (PI. 48, fig. ',) lias proportionally 

 also greatly increased in width from above the ambitus to the fourth or 

 fifth plate beyond it. The increase in width is due to the lateral growth 

 of the median ambulacral space between the poriferous zones. Above the 

 ambitus the large primary ambulacral plates now form a very regular verti- 

 cal series, while on the actinal side, owing to the great height of the 

 primary plates, the inner angle of each plate projects beyond the median 

 line. In the ambulacral plates immediately above and below the ambitus 

 the primary plates are excluded from the interambulacral edge, owing to 

 the great increase in height of the third poriferous plates (PI. 48, fig. J f ). 

 This great irregularity in the arrangement of the poriferous zone is charac- 

 teristic of all Echinothuriae. 



In a specimen 75 mm. in diameter (Pis. 48, fig. 5; 31, figs. 1, ,.') above 

 the ambitus a very narrow side of the primary plate again reaches the in- 

 teranibulacrum, the third plate being higher than the primary, and the 

 second pushed back of the third. Much the same proportions exist in the 

 arrangement of the ambulacral plates of a specimen of 120 mm. in diameter 

 (Pis. 48, fig. 6; 31, fig../). 



In the specimen of 53 mm. (PI. 48, fig. 4) the vertical rows of primary 

 tubercles are not readily traced ; they are fairly distinct on the actinal 

 side, where the primary tubercles are larger ; on the abactinal side, on the 

 central part of the median interambulacral belt plates, are found a few 

 large miliaries arranged in horizontal rows, and a few miliaries on the 

 outer edge of the poriferous zone. But in larger specimens the primary 

 vertical rows again become prominent, as in a specimen of 75 mm. in diameter 

 (Pis. 48, fig. 5; 31, figs. Y. .'), and in a specimen of 120 mm. (Pis. 48, fig.'';; 

 31, fig. 3); but in both, the primaries arc larger on the actinal side. This 

 contrast between the primaries of the actinal and abactinal sides is still more 

 marked in the interambulacral areas, where with increasing size the scro- 

 bicular areas become well limited (PI. 49; see also Pis. 30; 31 ; 35-38). 



In a specimen of 53 mm. the apical system measured 12.5 mm. in 



