GEODIA AMPHISTRONGYLA. 



179 



proximal portion and straight in their distal portion. Their chords are 26-50 [x 

 long and enclose angles of 25-41° with the rhabdome. 



I found one remarkable anatriaene-derivate (Plate 20, fig. 4) which possesses, 

 besides the three recurved anatriaene-clades and the rhal^dome, a fifth ray, 

 directed obliquely upwards. This spicule appears as a mesanatriaene with 

 oblique epirhabd. 



The choanosomal oxyasters (Plate 20, figs. 12-16, 26-30a) have no central 

 thickening or a hardly perceptible one, and usually straight and simple, equally 

 distributed rays. Very rarely one or more rays are either curved (Plate 20, figs. 

 13) or bifurcate. The rays are conical, smooth at the base, and spined in their 

 distal part. In the thin-rayed, prol)ably young, oxyasters the spines are so 

 small as merely to give to the rays, when examined with the highest powers, a 

 slightly rough appearance. In the thick-rayed, presumably adult, forms the 

 spines are usually large, sometimes nearly 1 /{ long. They arise vertically or 

 slightly obliciuely from the rays, and are in the latter case directed upwards, 

 towards the tip of the ray. All the spines which were large enough to be dis- 

 tinctly made out, were straight. Recurved, claw-like spines were not met with. 

 Sometimes (Plate 20, figs. 13, 16 right upper corner) the spines are massed at the 

 tip of the ray, more frequently (Plate 20, figs. 14, 15, 16 middle, left) they are 

 sparsely scattered over its distal two thirds. The ends of stout rays are often 

 crowned by one particularly large, terminal spine. These oxyasters have from 

 five to nine rays and measure 20-30 /t in total diameter. The rays are 10-15 /« 

 long and at the base 0.8-2.1 n, usually 1.5-1.8 thick. A correlation between 

 the number of the rays and the dimensions of the aster is not noticeable. 



The oxysphaerasters (Plate 20, figs. 26b, 27b, 30b) have a spherical centrum 

 0-7 fj. in diameter, from which from fourteen to eighteen stout, straight, conical, 

 and sharp-pointed, equally distributed rays arise radially. The rays are 6-11 

 H long and, at the base, 2-2.8 /j. thick. From their distal part spines, which are 

 usually quite large, arise. The base of the ray is always free from spines. A 

 rather regular verticil of particularly large spines, situated some distance below 

 the end of the ray, is sometimes observed. The total diameter of these spicules 

 is 19-28 A correlation between the number of their rays and the dimen- 

 sions of the aster is not discernible. 



The small strongylosphaerasters (Plate 20, figs. 28c, 29c, 34-36) consist of 

 a spherical or somewhat irregular centrum, 2.2-3 //, rarely as much as 4 /i in diam- 

 eter, from which from seven to twelve rays arise. These are regularly distributed 

 and in the same aster equal in size, or, more rarely, unequal in position and 



