136 GEODIA AGASSIZII. 



crease in size and begin to assume the flattened ellipsoidal appearance of the 

 full-grown ones (Plate 32, figs. 29-31). Then the basal parts of the protmding 

 rays coalesce farther and spines begin to grow out from the margin of their 

 terminal face (Plate 30, figs. 15, 16). These spines at first appear as small 

 rounded knobs. Later (Plate 30, fig. 17) the rays are thickened terminally 

 and the spines grow in length. They are in such young sterrasters very slender 

 and do not attain their full thickness for some time. 



The earl}- stages of the sterroids differ from those of the normal sterrasters 

 described above by the rays composing them being not nearly so niuncrous and 

 usually also not so regularly arranged. In accordance with the smallness of 

 the number of the rays the angles between them are much larger than in the 

 young forms of the normal sterrasters. The thickening and concrescence of 

 the basal parts of the rays, which in the latter lead to the early formation of 

 a solid centrum, here therefore does not have this effect until a very much later 

 stage. Young stages of the thick- and few-rayed sterrasters 60-70 /( in diam- 

 eter (Plate 32, figs. 13-20), which correspond to the stages of the normal sterr- 

 aster represented on Plate 32, figs. 29, 30, accordingly have a much smaller 

 centrum and much longer cylindroeonical, terminally rounded protruding rays. 

 As stated above these rays are often irregularly distributed and the angular 

 distances between them are very unequal. Rays of such young sterrasters 

 standing particularly close together coalesce as early as all rays of the normal 

 sterrasters do, whereby the irregularity in the appearance of these spicules is 

 greatly enhanced (Plate 32, figs. 15, 16, 19, 20). The distal parts of the rays 

 cover themselves with numerous small spines, which later grow in size. Sub- 

 sequently, through the continued thickening and concrescence of the ba.sal 

 parts of the rays, the centrum increases in size (Plate 32, figs. 21-26). Finally 

 sterroids are formed, the centra of which are as large as the centra of the normal 

 sterrasters, the surface of which, however, bears much fewer and usually thicker 

 rays covered with a much larger number of spines. While the rays of the 

 normal sterrasters are nearly always equal in length, the rays of the sterroids 

 are occasionally unequal. In such sterroids with rays unequally long the con- 

 crescence may reach up to or even beyond the shorter rays, which are then 

 totally enveloped by the mass of the centrum, their positions being indicated 

 in the full-grown sterroid only by the groups of spines on the parts of the sur- 

 face of the centnim free from protmding rays. In this way irregular few- and 

 thick-rayed sterroids like the one represented on Plate 32, figs. 27-28 are formed. 



The sterroids differ from the normal sterrasters accordingly not onl}' when 

 full grown but also, and even to a greater degree, when young. 



