260 
HYALONEMA (PRIONEMA) AGUJANUM. 
It is different with the amphidiscs with smooth teeth, stouter shaft, and 
broader anchors. There is a great gap in the length frequency-curve of these 
spicules in all the three forms: — in the var. tenuis, form A, curve between 80 and 
225 m; in the var. tenuis, form B, curve between 78 and 200 /x; and in the var. 
lata curve between 64 and 207 /x. This clearly divides these spicules biometri- 
cally into two groups: — macramphidiscs over 200, and micramphidiscs under 
80 /x in length. 
Besides one well-pronounced main elevation each of the three length fre- 
quency-curves of the macramphidiscs shows only a quite insignificant secondary 
elevation. The macramphidisc group can therefore be considered as fairly 
homogeneous. 
The micramphidiscs on the other hand show clearly pronounced gaps in the 
length frequency-curves; in the var. tenuis, from A, curve between 30 and 51 /x; 
in the var. tenuis, form B, curve between 33 and 47 n; and in the var. lata curve 
between 26.8 and 47 /x. These gaps divide them into two distinct groups, one 
comprising the micramphidiscs over 47 /x in length, the other the micramphidiscs 
under 33 m in length. For this reason, and because the former are also distin- 
guished from the latter by their shafts, which in the larger ones are provided 
with a relatively very large central tyle, and which in the smaller ones are not 
thickened at all, or only slightly so at or near the centre, I divide the micram- 
phidiscs into two secondary groups : — large micramphidiscs with well-developed 
central tyle, and small micramphidiscs with no central thickening or only a 
slight one. 
Thus four kinds of amphidiscs are to be distinguished: — macramphidiscs, 
serrated amphidiscs, large micramphidiscs, and small micramphidiscs. 
The normal macramphidiscs (Plate 73 , figs. 1-7; Plate 75 , figs. 3-21; Plate 
77 , figs. 1,9, 10; Plate 78 , figs. 12-15) have a straight and stout cylindrical shaft, 
slightly thickened at or near the middle to a central tyle. A verticil of stout 
and short, distally attenuated, truncate spines arises from this tyle. The 
number of spines forming the verticil is variable but never great, most frequently 
four to eight. The verticil is regular or irregular. Its irregularity is usually 
slight, rarely considerable. In the latter case there are more than eight spines. 
The remaining parts of the shaft are either quite smooth or they bear only a few 
scattered spines nearly as broad as the spines of the central tyle, but usually 
much shorter. 
The terminal anchors are composed of eight teeth quite uniformly curved 
throughout (Plate 73 , figs. 4, 6) or more strongly bent at the ends than elsewhere 
(Plate 73 , fig. 5). Their curvature is such that the end-parts of the teeth are 
