64 



GEODIA VARIOSPICULOSA. 



cladomal end, 4-6 n thick at the thickest point, and 1.5-2 // thick at the 

 rounded acladomal end. The chords of the clades are 5-12 n long and enclose 

 angles of 3S°-54° with the axis of the rhabdome. In var. intermedia the 

 rhabdomes of these spicules are 205-560 /< long, 1-3.5 n thick at the cladomal 

 end, 3-7.5 /< thick at the thickest point, and 1-4.5 {i thick at the rounded, 

 acladomal end. The clade-chords are 3-13 n long and enclose angles of 40°- 

 52° with the axis of the rhabdome. 



The large choanosomal oxyasters (Plate 18, fig. 1, 2a, 3, 4, 5d, 6, 7b, 10a, b, 

 12, 14b, 15-20, 22a, b, 25a, b, 27a, b; Plate 19, figs. 25-30) have no central 

 thickening and are composed of fairly concentric but often not quite regularly- 

 distributed rays. The rays are straight, conical, usually very blunt, truncate, 

 rarely pointed, and everywhere, except at the proximal (central) end, covered 

 with spines. The size of these spines is variable. Sometimes they are so 

 small as to be hardly discernible, sometimes they are large, 1 // or more long. 

 When large enough to be distinctly seen, they show an increase in size from the 

 base to the tip of the ray. These spines rise vertically from the ray and appear 

 to be bent back at the end towards the centre of the aster in a claw-shaped 

 manner. There are usually from one to eight rays. In the form where only 

 one ray is developed, short, rounded, knob-like rudiments of two or three other, 

 reduced rays are observed (Plate 18, figs. 5d, 0; Plate 19, fig. 28). Such ray- 

 rudiments also occur in most of the diactine and in some of the triactine forms. 

 The terminal rounded faces of these ray-rudiments are densely covered with 

 large spines (Plate 19, fig. 28). The monactine oxyasters appear as blunt 

 tylostyles with irregularly lobate tyles. In the diactine forms the fully de- 

 veloped rays seem never to extend in a straight line, the angle enclosed by 

 them being 60°-140°, always much less than 180°. Thus these spicules appear 

 as more or less widely opened compasses. The three rays of the triactine 

 oxyasters may he situated in a plane and regularly arranged so as to enclose 

 angles of 120° (Plate 18, fig. 10a); or they may not be situated in a plane and 

 form the edges of a low triangular pyramid which may be regular (Plate 18, 

 fig. 2a) or irregular (Plate 18, fig. 16); or finally they may be so arranged 

 that two lie in a straight line to which the third is vertical or oblique (Plate 

 18, fig. 4). Among the asters with from four to eight rays both regular forms 

 with fairly equal angular intervals between the rays (Plate 18, figs. 1, 15, 19), 

 and irregular forms with unequal angular ray-intervals (Plate 18, figs. 17, 18, 

 20) are met with. In these tetr- to oct-actine asters the rays are, in the same 

 aster, usually of equal size. The tri- to hex-actine asters are far more numerous 

 than those with one or two, or seven or more rays. 



