ERYLUS ROTUNDUS. 

 DIMENSIONS OF TRIAENES. 



301 



The acanthtylasters (Plate 6, figs. 14b, 15, 16; Plate 7, figs. 52 54b, 56b, 

 57b, 58, 59, 75b, 76b) have from two to fourteen concentric, regularly dis- 

 tributed rays. Two-rayed acanthtylasters are rare and have been observed 

 only in var. cidaris. Also the three-rayed, which have been found in var. 

 typica, form B, var. megarhabda, form A, and in var. cidaris, are not frecjuent. 

 Four- to eight-rayed acanthtylasters are abundant in all the forms. Acanthtyl- 

 asters with more than eight rays appear to be most frequent in var. typica, 

 form B, and in var. megarhabda, form A. The acanthtylastei-s measure 12-31 /( 

 in total diameter. Those of var. cidaris are somewhat smaller than those of the 

 other two varieties. The size of the acanthtylasters is on the whole in inverse 

 proportion to the number of their rays, those with from two to six rays being 

 16-31, those with from seven to nine rays 14-24, and those with from ten to 



