GEODIA MESOTRIAENA. 



109 



teen to twenty-five straight, conical, sharp-pointed, and uniformly distributed 

 rays arise radially. The diameter of the centrum is from one seventh to one 

 third of the diameter of the whole aster. The rays are very spiny, particularly 

 in their distal part, 7-12 /i long, and 1-3 // thick at the base. 



The small stro7igylosphaerasters (Plate 24, figs. 5-6c, 7d, 10-14b, 16-18, 20, 

 21b, 32) have from six to twenty concentric rays, usually equal in size, and 

 quite regularly distributed. Rarely such asters with unequal rays occur. In 

 these there are three or more long rays, and a number of more or less shortened, 

 rudimentary ones. The rays are cylindrical or cylindroconical, usually rounded 

 off terminally, covered with small spines, without the centium 2-7 ft long, and 

 0.5-2.5 fi thick. The centrum is 2-6 /«, and the whole aster 6-14.5 /x in diameter. 

 The small strongylosphaerasters of var. pachana have more numerous and more 

 slender rays than the strongylosphaerasters of the other two varieties. 



The rare large strongylosphaerasters (Plate 24, figs. 7b, 9a, 21a), transitional 

 between the small strongylosphaerasters and the sterroids, have numerous, 

 usually cylindroconical rays densely covered with large spines. Their centmm 

 is from one third to a half of the whole aster in diameter. Their total diameter 

 is 16-33 fi. 



The rare steiroids (Plate 24, fig. 8a), wliich have been observed in var. 

 megana only, have a very large centrum, two thirds or more of the whole aster 

 in diameter, from which very numerous, short and stout, cylindrical rays arise. 

 These are 5-8 /x long and 4-6 fi thick. Their sides are smooth, their convex 

 terminal faces covered with numerous spines. These asters measure 39-58 fi 

 in total diameter. 



DIMENSIONS OF STERRASTERS OF GEODIA MESOTRIAENA. 



The sterrasters (Plate 22, figs. 1-7; Plate 23, figs. 24, 25; Plate 26, figs. 1-11) 

 are flattened ellipsoids 92-125 /x long, 78-107 /( broad, and 67-82 /t tliick. Their 

 average length and average thickness is nearly the same in all the three 



