160 



GEODIA BREVIANA. 



2.3 p. thick, and arc 19-25 ji (Lambe, 1893, 3-13, according to his figures 

 11-15 /<) in total diameter. 



The rare thin-rayed oxyasters which, as above stated, may be foreign, have 

 from nine to fourteen rays and no centrum. The rays are 3-11 fx long, and very 

 thin, only 0.25-0.7 /t thick at the base. They are not very much attenuated 

 towards the end and bear spines which sometimes form terminal verticillate 

 clusters, in which case these asters appear as acanthtyl asters. The total diame- 

 ter of these asters is 7-23 fi. If they are not foreign, they may be young forms 

 of the thick-rayed oxyasters described above. 



The large oxysphaer asters (Plate 35, figs. 21c, 25, 26) have a spherical cen- 

 trum, 7-9 fi in diameter, from which from twenty-three to twenty-seven straight, 

 conical, and sharp-pointed, usually regularly distributed rays arise radially. 

 These rays are (without the centrum) 6-8 fi long and 1-2.7 jj. thick at the base. 

 They are covered with a greater or smaller number of good-sized spines. The 

 rays of the oxysphaerasters with twenty-six or twenty-seven rays are basally 

 only 1-2.2 /j., those of the oxysphaerasters with twenty-three or twenty-four rays 

 basally 2.5-2.7 p. thick. This incUcates that there is an inverse proportion 

 between the ray-number and the ray-thickness. The whole aster is 14-21.5 

 in diameter. In the specimen described by Lambe these asters have up to 

 thirty rays 1-1.7 /< thick, the centre is 3-5.5 the whole aster 12-18 fi, in diame- 

 ter. They are not mentioned by Lambe (1893). 



The small strongylosphaerasters (Plate 35, figs. 8-13, 18-22b) have a centrum 

 3-5.5 /< in diameter and usually from thirteen to twenty-one, very rarely only 

 seven or three rays. The rays are generally radial and regularly distributed, 

 rarely arranged irregularly. Such an irregularity is chiefly observed in the 

 rare few-rayed forms which are evidently derivates of the ordinary many-rayed 

 ones, produced by the suppression of a smaller or greater number of rays. The 

 rays are cylindrical or cylindroconical and truncate or terminally rounded. Their 

 basal parts are smooth, their distal parts covered with a number of good-sized 

 spines. The rays are 1.8-5 // long and 0.6-1.7 /x thick; the whole aster measures 

 7-12 fj. in diameter. A correlation between ray-number and spicule-size is not 

 pronounced. In the specimen described by Lambe these asters have from four- 

 teen to twenty-five rays, 0.5-1.2 p. thick, the centre is 2-4 p, the whole aster 

 6-9 fi, in diameter. They are not mentioned by Lambe (1893). 



The sterrasters (Plate 35, fig. 23) are flattened ellipsoids, 87-105 p long, 

 80-98 p broad, and 70-77 p thick. The proportion of length to breadth to thick- 

 ness is on an average 100 : 91 : 74. In the specimen described by Lambe the 



