HYALONEMA (HYALONEMA) OBTUSUM. 
165 
In var. robusta they are larger, sometimes 1.1 mm. long and 20 n thick. A slight 
central thickening (tyle) with an axial cross in the interior can usually be made 
out, particularly in the smaller uncinates. This is generally about 0.5, rarely 
as much as 1 m thicker than the adjacent parts of the spicule. The spicule is 
covered with slender spines, all strongly inclined in the same direction. Near 
the end from which these spines diverge, they are rather numerous, towards the 
other end they become very scarce. So far as I could make out these spines 
consist of a rather broad conic basal part and a fine, exceedingly slender, neeclle- 
like end-part. The basal conic part arises steeply from the shaft and bends 
round above, where it passes into the fine end-part, so that the latter comes to 
lie nearly parallel to the shaft. 
The large stalk-spicules (Plate 40 , figs. 21, 22), in var. robusta 8 mm. below 
their upper ends, where they are all broken off, are 40-720 /i thick. The empty 
spaces previously occupied by them in var. gracilis have a maximum width of 
900 /d. The upper ends of these spicules of var. robusta are curved, the curvature 
increasing towards the (upper) end. The axial thread is for the most part 3-4 m 
thick. It does not lie centrally, but describes a spiral line around the mathe- 
matical axis of the nearly cylindrical spicule. It is by no means a simple cylindri- 
cal thread. Some parts of it (Plate 40 , fig. 21) are uncinate-like, covered with 
strongly inclined spine-like processes directed upwards, others (Plate 40 , fig. 22) 
are thickened, quite irregular, and attain 20 n in transverse diameter. 
In both varieties the regular microhexactines (Plate 35 , figs. 14, 15, 17a, 18, 
19; Plate 40 , figs. 6, 7, 20b) measure 42-80 m in diameter. The six rays of the 
same spicule are fairly equal, and regularly arranged. The chords of the rays 
are 20-43 /j. long. The rays themselves are 1.5-2. 2 m thick at the base, gradually 
and uniformly attenuated distally to a fine point, and covered with very minute, 
vertically arising spines. The basal parts of the rays are nearly straight, the 
distal parts strongly curved through an angle usually a little over 90°. The 
direction of curvature of the end-part of each individual ray is generally opposite 
to that of the end-part of the ray opposite it in the same axis. 
The microhexactine-derivates (Plate 35 , figs. 20-22; Plate 40 , figs. 8-15, 20c) 
represent two series of forms. One begins with microhexactines in which the 
two rays lying opposite in the same axis are longer than the other four, and ends 
with centrotyle diactines. The other begins with micropentactines with equal 
rays, and ends with style monactines. In var. robusta forms of both series are 
rather frequent; in var. gracilis hardly any but diactine forms, with the two fully 
developed rays opposite in the same axis, have been observed. 
First series of microhexactine-derivates. One of the microhexactines, with 
