166 HYALONEMA (HYALONEMA) OBTUSUM. 
two opposite longer, and four shorter rays, with which the first series commences, 
is represented in Plate 40 , fig. 9. This spicule is 117 n in length. Forms still 
farther removed from the regular microhexactine are produced by a further 
reduction of the four shorter rays of such a spicule. The reduction of the four 
shorter rays is either unequal or more or less equal. In the first case pentactines, 
tetractines, and triactines (Plate 40 , fig. 11) with two opposite longer rays, and 
three, two or only one shorter, are produced ; in the second case forms like those 
represented on Plate 35 , figs. 20-22, and Plate 40 , figs. 12, 13, and 20c. In the 
extreme forms of this series all that remains of the shorter rays is a slight tyle 
(Plate 35 , fig. 22; Plate 40 , fig. 12). It is to be noted that a distinct increase 
in size of the two opposite, developed rays is, in these spicules, associated with 
the reduction of the four other rays. Such diactine microhexactine-derivates 
are, particularly in var. gracilis, more numerous than any of the others. They 
are in both varieties 1 56-204 n long, but in var. robusta considerably stouter than 
in var. gracilis, the basal parts of their properly developed rays being in the former 
1.5-4 tx, while in the latter only 1.5-2. 5 m thick. The fully developed rays of 
these spicules are gradually attenuated to fine points, straight in their basal part 
and curved at the end. The reduced ones are straight throughout, cylindrical 
or cylindroconic, terminally rounded, and reach 6 n in length. The terminal 
curvature of the fully developed rays is not so great as in the rays of the regular 
microhexactines, nor is its direction generally opposite. 
To the second series of microhexactine-derivates belong the spicules repre- 
sented on Plate 40 , figs. 8, 10, 14, and 15. The first (fig. 8) of these is a pentactine 
with equal rays, 100 ^ in diameter. The second (fig. 10) is a compass-shaped 
diactine. It consists of two fully developed rays, 47 n long, the basal parts of 
which enclose a right angle; and the insignificant rudiments of two other rays 
opposite to the two fully developed ones. The third and fourth (figs. 14, 15) 
are monactines. Such monactines are more frequent than the other forms of 
this series. They are 73-86 n long. Their single fully developed ray is 2.5-4 n 
thick at the base and tapers gradually to a fine point. It is straight in its basal 
part but strongly curved, through an angle of about 120°, in its distal part. 
These spicules are, like the regular microhexactines, covered with minute 
spines. In the larger ones the spines are more conspicuous than in the smaller 
ones, the size of the spines being, generally speaking, proportional to the thick- 
ness of the ray from which they arise. 
The pachymicrohexactines (Plate 39 , figs. 7-10) are rather rare, and have 
only been found in the basal part of var. gracilis. I consider them as hypertrophic 
