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 raj^s. 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 ^ 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 n long. The regular 

 microhexactines of the two forms of var. tenuis are 100-180 yu in diameter, of var. 

 lata 110-240 ix. The basal thickness of the rays is in the former 3-5 n, in the 

 latter 3-7 fx. 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 ajufhidiscs were examined biometrically in the usual manner. I 

 measured 238 of var. tenuis, form A; 66 of var. tenuis, form B; and 142 of var. 

 lata. To make these three sets of measurements directly comparable I multi- 

 plied the numbers of amphidiscs of the same length-category of \?a\ 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 amphidiscs are to be distinguished: — 

 amphidiscs with serrated anchor-teeth, more slender shafts, and narrow anchors; 

 and amphidiscs with smooth anchor-teeth, stouter shaft, and broader anchors. 

 The amphidiscs of the first group, which I designate serrated amphidiscs, vary 

 very considerably in size, their length ranging from 90 to 415 n. The curves 

 representing the frequency of the serrated amphidiscs of different lengths show 

 numerous ups and downs, thus indicating that the serrated amphidiscs 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 amphidiscs 

 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 measiu-ements than the curves 

 of var. tenuis, form B, and var. lata, I refrain from sulidividing the serrated 

 amphidiscs into subgroups. 



