GEODIA MEDIA 



203 



centrum are subject to great variations. In the dermal sphaerasters most 

 frequent in the digitate form the diameter of the centrum is always more than 

 half and usually less than two thirds of the diameter of the whole aster. The 

 rays of these asters are cylindrical or cylindroconical, truncate or rounded off at 

 the end, and covered with spines, chiefly in their distal portions. The size of the 

 spines is proportional to the thickness of the rays. In the dermal sphaerasters 

 most frequent in the massive form the centrum is relatively a Uttle smaller, its 

 diameter in not a few being less than one third of the total diameter of the spicule. 

 The rays of these asters are conical or cylindroconical, rounded off or, more 

 rarely, pointed at the end (Plate 17, figs. 1-3, 13, 14, 16, 17), and covered with 

 spines, chiefly in their distal parts. Of course both the oxyastrose and the 

 strongylastrose kinds of dermal sphaerasters occur in both forms. The dif- 

 ference lies in the fact that the strongylastrose forms are much more prevalent in 

 the digitate than in the massive form. In Bowerbank's type, these spicules 

 have from six to eighteen rays, which seem, on the whole, somewhat stouter 

 than those of the small strongylosphaerasters of the "Albatross" specimens. 

 The total diameter of the aster is 5.5-8 fx, that of the centrum 2-3.5 p.. Sollas 

 gives their diameter as 6 ^. 



Besides these a few other forms of asters, which may be considered as 

 sphaeraster-derivates, are found in the dermal membrane. One of these aster- 

 forms has a very large centrum and exceedingly short and thick rays, the latter 

 appearing as rounded knobs on the surface of the relatively overgrown centrum. 

 In another form the centrum is very much reduced and there are only five or 

 six cylindrical rays thickened at the end. The basal and central, cylindrical 

 parts of the rays of these asters are smooth, their terminal thickening covered 

 with numerous spines. Finally, there are sphaerasters the rays of which are not 

 concentric. These appear as metasters or ataxasters. 



The normal stcrrasters (Plate 16, figs. 15, 18, 19; Plate 17, figs. 21c, 22c) are 

 flattened ellipsoids, broad oval, sometimes nearly circular, in outline. The 

 sterrasters of the digitate form are 84-98 [x long, 73-86 /( broad, and 60-65 /x 

 thick; those of the- massive form larger, 89-105 /( long, 83-90 /< broad, and 

 about 62 /J. thick. The proportion of the length to the breadth to the thickness 

 is in both on an average about 10:9:7. With the exception of those surround- 

 ing the umbilicus, the rays protruding over the surface of the sterrasters are 

 2.5-3 /i thick and about 2 /t apart. They bear terminal verticils of usually from 

 four to six stout lateral spines, many of which are curved. The rays surround- 

 ing the umbilicus (Plate 16, fig. 15) have a somewhat oval transverse section 



