216 



GEODINELLA ROBUSTA. 



different in the three varieties, var. carolae having the smallest, and var. mega- 

 sterra the largest ; var. megadada being in this respect intermediate between the 

 other two. In var. carolae all or nearly all the sterrasters are flattened ellip- 

 soids, the proportion between the three axes being about 5:7:9 (Plate 2, fig. 

 5). In the other two varieties most of the sterrasters have a similar shape 

 (Plate 2, figs, la, 2a, c); but we find in these, among the ordinary, ellipsoidal 

 sterrasters, also a good many flattened, three-lobed ones (Plate 2, figs, lb, 2b). 

 The ellipsoidal sterrasters are in var. carolae 180-195 /i long, 130-160 p. broad, 

 and 80-115 /j. thick; in var. megadada 190-217 /j. long, 160-190 /i broad, and 

 105-125 fi thick; in var. megasterra 220-237 /( long, 165-200 p. broad, and 120- 

 130 [J. thick. The three nearly equal maximum diameters of the three-lobed 

 sterrasters of the two last-named varieties are nearly or quite as long as the 

 longest diameter of their ellipsoidal sterrasters. In the specimen of var. carolae 

 from Naha Bay I found two tetra-lobed sterrasters. 



The centre of the sterraster is, as Thiele has already noticed in another 

 species, Geodinella (Geodia (?)) cyli7id7ica,^ surrounded by granules the refrac- 

 tive index of which differs from that of the silica in which they are imbedded. 

 These granules form a hollow, spherical cluster 6-8 p in diameter (Plate 3, 

 figs. 7a, 9a). Rather to my surprise I found that in the three-lobed sterr- 

 asters the position of these pericentric granules is the same as in the ellip- 

 soidal ones. The siliceous substance surrounding this cluster of granules 

 shows the usual radial structure. The individual granules often appear to be 

 onion shaped and produced distally in a radial process, forming one of the radial 

 lines which give the radially striated appearance to the siliceous substance of 

 the sterraster. Sections, optical and other, through the sterrasters show that 

 many of them are not only radially striated but also paratangentially stratified, 

 one or two, very conspicuous limits (Plate 3, figs. 7, 9b) between the super- 

 posed zones being distinctly visible. These limits are concentric and parallel 

 to the outer surface. The radial striations pass continuously through them. 



The umbilicus (Plate 3, figs. 1-3, 7c, 9c; Plate 4, fig. 13a) hes in the 

 centre of one of the broad faces of the sterraster. It is generally a caliculate pit 

 15-25 n deep; its circumference (mouth) is oval, 17-20 // broad and 23-30 /ilong. 

 Proximal continuations of the rays surrounding the umbilicus project into the 

 umbilical pit and form longitudinal (radial) ridges on its flanks. The bottom 

 of the pit appears rough. The remainder of the surface of the sterraster is 

 covered by freely projecting rays (Plate 2, figs. 1, 2, 5; Plate 3, figs. 1-3, 7, 9; 



' Zoologica, 1898, 24, p. 13. 



