50 



PANAMIC DEEP SEA ECHINI. 



part of the actinal system. In still younger specimens, as in S. varispina, 

 these plates cover the whole actinal system (PI. 21, fig. /). The rest of the 

 actinal system is covered with plates irregularly shaped and irregularly 

 arranged (PI. 16, figs. •>', .7). 



'Plie primary radioles are usually cylindrical, marked for the great 

 development of the milled ring (PI. It), fig. 7) forming a deep groove round 

 the base of the larger radioles ; these vary greatly in appearance: some 

 of them are covered with fiat, sharp spinules arranged in irregularly concen- 

 tric rings round the shaft (PI. 19, fig. 7), which has an exceedingly fine 



6 mui. 



Fig. 83. Salenia miliaris. 



longitudinal striation ; in others the radioles have lost the spinules, and 

 the whole shaft retains only its delicate longitudinal lines ; while again the 

 striations may coalesce, forming a coarser fluting with traces on the sides 

 of the lines of sharp spinules which characterize some of the more flattened 

 radioles (PI. 19, fig. 8). The radioles on the actinal side are often flattened, 

 even spear-shaped, somewhat concave, with larger serrations on the edges, 

 and recalling to a certain extent the serrated actinal spines of Porocidaris. 

 This tendency to become spathiform at the extremity is also seen in some 

 of the largest primary radioles. The longest radiole observed was 85 mm. 

 long in a specimen of 18 mm. in diameter. In younger specimens the 



