258 HYALONEMA (PRIONEMA) AGUJANUM. 
Among the microhexactines two kinds can be distinguished :— regular forms 
with equal rays, and irregular forms with one ray or two opposite rays longer 
than or otherwise different from the others. 
The regular microhexactines (Plate 72, figs. 16-18; Plate 76, figs. 1-3; Plate 
78, figs. 5-7) usually have perfectly straight rays. Very rarely one or the other 
of the rays is somewhat curved. The rays are conical and sharp-pointed. They 
bear conical, sharp-pointed spines (Plate 76, fig. 1). The spines on the proximal 
part of the rays are sparse, vertical, and about 0.6 w long. Distally the spines 
become more numerous, inclined backwards toward the centre of the spicule, 
and smaller; those a short distance below the end are 0.3 » long. The regular 
microhexactines of the two forms of var. tenuis are 100-180 u in diameter, of var. 
lata 110-240 ». The basal thickness of the rays is in the former 3-5 yu, in the 
latter 3-7 ». The centre, particularly of the larger microhexactines of var. 
lata, is often distinctly thickened. 
The irregular microhexactines are to be considered as forms transitional 
between the canalar pinules and the regular microhexactines, and in respect to 
shape and size intermediate between these. 
The amphidiscs were examined biometrically in the usual manner. I 
measured 238 of var. tenuis, form A; 66 of var. tenuis, form 6; and 142 of var. 
lata. To make these three sets of measurements directly comparable I multi- 
plied the numbers of amphidises of the same length-category of var. tenuis, 
form B, with 238:66 = 3.606, and of var. lata with 238: 142 = 1.677. The 
numbers thus obtained are the ones used in constructing Figure 12. 
Morphologically two main groups of amphidises are to be distinguished : — 
amphidiscs with serrated anchor-teeth, more slender shafts, and narrow anchors; 
and amphidises with smooth anchor-teeth, stouter shaft, and broader anchors. 
The amphidises of the first group, which I designate serrated amphidiscs, vary 
very considerably in size, their length ranging from 90 to 415 uw. The curves 
representing the frequency of the serrated amphidises of different lengths show 
numerous ups and downs, thus indicating that the serrated amphidises of differ- 
ent size differ in frequency. The irregularities of these curves are, however, 
hardly of a kind to allow of a distinction of different kinds of serrated amphidises 
according to their size. This is particularly noticeable in the curve of var. 
tenuis, form A. And as this curve is the most reliable one, on account of its 
being based on a much larger number of individual measurements than the curves 
of var. tenuis, form B, and var. lata, I refrain from subdividing the serrated 
amphidises into subgroups. 
