168 



GEODIA OVIS. 



as large thin-rayed oxyasters. 2. Large asters, here described as large thick- 

 rayed oxyasters, with very stout, conic, pointed rays, in which the length of 

 the rays is less than five times as great as their basal thickness. 3. Smaller 

 asters, here described as small thick-rayed asters, with stout, truncate, blunt or 

 pointed rays. 



The large thin-rayed oxyasters (Plate 41, figs. 3, 9b, 12, 15, 19; Plate 42, 

 figs. 13b, 14b, 17b, 21b, 22b) are destitute of a central thickening and have 

 from three to ten usually quite concentric and simple, straight, conical, sharp- 

 er blunt-pointed rays. The proximal third of the rays is smooth, the distal two 

 thirds are covered by spines which are usually large and conspicuous (Plate 41, 

 figs. 15, 19), more rarely so small as merely to give to its distal part a slightly 

 roughened appearance (Plate 41, fig. 3). The rays are 11-18 /« long and, at the 

 base, 1-3.2 /t thick, the total diameter of the aster being 20-34.5 ji. 



The many-rayed large thick-rayed oxyasters (Plate 41, figs. 10b, lib, IB- 

 IS, 20; Plate 42, figs. 12-15a, 20a, 21a) appear, in consequence of the con- 

 crescence of the basal parts of the exceedingly thick rays, as sphaerasters ; in 

 those with few rays, however, no trace of a central thickening can be detected. 

 The rays of the many-rayed forms are fairly concentric and regularly distributed 

 (Plate 41, fig. 16; Plate 42, figs. 12-15a, 20a, 21a), while those of the few-rayed 

 forms are sometimes eccentric and, as a rule, not regularly distributed (Plate 

 41, figs. 10b, lib). The rays are straight, conic, and usuaUy very sharp pointed. 

 Most of them are simple but in a good many of these asters one or even two of 

 the rays are bifurcate (Plate 41, figs. 10b, lib). The extreme tip and the basal 

 portion of the rays are usually quite smooth, their remaining part covered with 

 large, vertically arising, terminally recurved, claw-like spines. In some of these 

 asters the spines are quite numerous, in others rather scarce and restricted to a 

 verticillate belt situated some distance below the end. These asters have from 

 four to nineteen rays, 14-24 /j. long and, at the base, 3-6.3 /t thick. In some of 

 the many-rayed forms the central thickening attains a diameter of 12 /x. The 

 total diameter of the aster is 28-45 fx. 



The small thick-rayed asters (Plate 41, figs. 1, 2, 4-8, 9a, 10a, 11a, 13, 

 14; Plate 42, figs. 16c, 21c) are without centrum or have a central thickening 

 up to 6 ptm. diameter. They have from six to fifteen concentric, regularly 

 distributed, and usually equal, more rarely unequal (Plate 41, figs. 13-14), 

 truncate, blunt or pointed rays. The distal parts of the rays are densely covered 

 with large spines which are not, as is the case in the large thick-rayed oxyasters, 

 restricted to an intermediate zone but extend right up to the tip of the rays. 



