192 



GEODIA ACANTHTYLASTRA. 



at the base, 1-2.2 p. thick. The distal parts of the rays bear a few rather large, 

 vertically arising spines, which usually form, some distance below the end, a more 

 or less pronounced verticil. Besides the spines forming this verticil, others, 

 situated more proximally, are not infrequently observed. The total diameter 

 of the oxysphaerasters is 12-16.5 p.. A correlation between size and ray- 

 number could not be detected. 



The oxysphaerasters which I consider as transitions to the small strongylo- 

 sphaerasters are similar to the true ones described above, have from ten to twenty- 

 five rays, and measure 6-9.5 p. in total diameter. Their centrum is 2 3 p, about 

 a third of the whole aster, in diameter. The conic, spined, and pointed rays are, 

 without the centrum, 1.5 3 p. long and, at the base, 0.6-1.3 p thick. 



The oxysphaerasters which might be considered as transitions to the oxy- 

 asters are also similar to the true ones, have from sixteen to twenty rays, measure 

 about 28 p in total diameter and have a centrum 5-6 p, from a sixth to a fifth of 

 the whole aster, in diameter. 



The acanthtylasters which are the chief choanosomal microscleres (Plate 45, 

 figs. 23-27a; Plate 47, figs. 1, 2, 3b, 4-6, 7b, 8) have from four to twelve 

 regularly distributed rays and usually a central thickening 1.7-3 p in diameter. 

 The rays are straight, cylindroconical, attenuated distally, and, at the base, 0.5-1.3 

 p thick. Rays over 1 p thick are found only in the larger, few-rayed acanthtyl- 

 asters. The rays are truncate ; often a small and slender terminal spine arises 

 from their terminal face. The rays bear a terminal verticil of protuberances, 

 which together form a conspicuous tyle 0.9-1.6 p in transverse diameter. The 

 individual protuberances forming this tyle, are so minute that it is difficult to 

 make out their shape. They are always more or less recurved in a claw-shaped 

 manner. Sometimes they appear as rounded knobs, sometimes as stout-pointed 

 spines and sometimes as more slender and longer branches, strongly curved 

 backward like the clades of anatriaenes. A few protuberances (spines) similar 

 to those forming the terminal verticil (acanthtyl) are often observed farther down 

 on the rays. The total diameter of the acanthtylasters is 11-22 p. It is in 

 inverse proportion to the number of the rays. Four- to five-rayed acanthtyl- 

 asters are 15-22 p, six- to seven-rayed 13.5-20.5 p, eight- to nine-rayed 11.5- 

 18 p, and ten- to twelve-rayed 10.5-15 p in diameter. 



The small strongylosphaerasters (Plate 45, figs. 24b, 25b, 27b; Plate 46, 

 figs. 1-7; Plate 47, figs. 3a, 7a) have a spherical centrum 1.5-2.7 p, rarely as 

 much as 3.4 p, from about one third to two fifths, seldom as much as one half 

 of the whole aster, in diameter. From this from fourteen to twenty-two quite 



