GEODTA ATAXASTRA. 



93 



from four to nine rays and measure 10-14.5 jx in diameter. The acanthtylasters 

 have no central thickening. Their rays are usually distributed fairly regularly, 

 cylindrical, 1-2 thick, and rounded at the end. The end, which usually appears 

 distinctly thickened, is densely covered with large spines, which are for the most 

 part directed outward. The central and proximal parts of the rays are quite 

 smooth. In the asters of this kind which I have found in var. angustana the 

 rays were always simple, while in several of the acanthtylasters of var. latana 

 some of the rays were terminally divided into two or, rarely, three short branches, 

 each provided with a special acanthtyle. 



The rare irregular sterraster-derivates , which have been observed only in var. 

 latana, consist of a central mass from which tufts of large, slender spines arise. 

 The central mass is either simple and spherical, or oval, in which case two 

 tufts of spines, lying nearly opposite each other, rise from it; or it is lobate, in 

 which case the distal convex face of each lobe is covered with spines. The 

 number of the lobes of the lobate form is from two to five. Sometimes the 

 lobes are separated by rather deep recesses. The spines are 4-8 // long, 

 the whole spicule measuring 21-50 ft in diameter. In their thmension and 

 in the shape, size, and position of the spines these spicules agree with young 

 sterrasters, and they make, on the whole, the impression of being early stages 

 of abnormal spicules of this kind. Abnormal sterrasters, which might be con- 

 sidered as their adult forms, have, however, not been observed. 



The sterrasters (Plate 44, figs. 36-38, 43-45) are abundant in both varieties. 

 They are flattened ellipsoids. Those of var. angustana are 65-98 /i long, 58-67 /j. 

 broad, and 47 57 ft thick, the average proportion of length to breadth to thick- 

 ness being 100 : 89 : 74. The sterrasters of var. latana are similar but smaller 

 and relatively thicker. They are 55-65 n long, 50-60 /« broad, and 47-50 /( thick, 

 the average proportion of length to breadth to thickness being 100 : 90 : 79. 



In the centre of the sterraster a cluster, about 4 /« in diameter, of small 

 granules is observed. Outside the umliilical area the rays protruding beyond 

 the surface are about 2.3 /( thick and have a circular or broad-oval transverse 

 section. The transverse section of the rays surrounding the umbilicus is greatly 

 elongated in a direction radial to the centre of the latter and measures about 

 2.4 by 7 fi. The terminal spine verticils are, in the rays away from the umbilicus, 

 composed of from six to ten, in those of the rays surrounding the umbilicus of 

 a much larger number of lateral spines. On some of the periumbilical rays I 

 counted as many as sixteen. The spines are broad, conical, and small, mostly 

 under 1 ^ in length. The terminal faces of the periumbilical rays are very 



