386 
HYALONEMA (SKIANEMA) UMBRACULUM. 
The larger macramphidiscs have very broad and rather short anchors, 
usually about a third of the whole spicule in length. The smaller have rela- 
tively much longer anchors, usually about half the whole spicule in length. 
Forms intermediate in respect to the proportion of the length to the breadth of 
the anchors connect the larger, shorter- and the smaller, longer-anchored 
kinds of these spicules. These intermediate forms are, however, far from 
numerous. 
The length frequency-curve in Figure 25 shows that the larger (short- 
anchored) and the smaller (long-anchored) macramphidiscs are very clearly 
distinguished biometrically. This biometrical distinction, together with the 
rarity of the forms transitional between the two morphologically, makes a sub- 
division of the macramphidiscs into two subgroups necessary; namely: — large 
macramphidiscs, larger forms with anchors usually about a third of the length 
of the whole spicule; and small macramphidiscs, smaller forms with anchors 
usually about half of the whole spicule in length. 
The part of the length frequency-curve pertaining to the micramphidiscs 
is divided, by a deep depression extending quite down to the 0-line, into two 
parts, one comprising the larger forms, 54-99 n in length, the other comprising 
the smaller forms, 16-38.7 n in length. Although the larger and the smaller 
of these spicules differ morphologically only in so far as the anchors are on the 
whole relatively broader in the former than in the latter; nevertheless I think 
it advisable to distinguish also in this main amphidisc-group two subgroups, 
namely, large micramphidiscs, comprising the larger forms with broader anchors, 
and small micramphidiscs, comprising the smaller forms with narrower anchors. 
The length frequency-curves pertaining to the large and small macramphi- 
discs are quite simple and have, each, only one summit; these two amphidisc- 
groups are obviously homogeneous. The curves pertaining to the micramphi- 
discs on the other hand have, each, two depressions, dividing each into three 
parts. Although this division is very well-marked, particularly in the small 
micramphidiscs, I do not propose further to subdivide these subgroups of amphi- 
discs because I was unable to detect any morphological differences between the 
respective amphidiscs to which the different elevations of the curves pertain. 
Thus I distinguish four kinds of amphidiscs in this sponge : — large and small 
macramphidiscs, and large and small micramphidiscs. 
The regular large macramphidiscs (Plate 102 , figs. 1, 2, 7, 8; Plate 103 , 
figs. 1-8, 14-23) are 110-280 n long, most frequently about 268.4 n long. The 
shaft is straight, cylindrical, smooth, and 17-26.5 m thick. The terminal anchors 
