HYALONEMA (OONEMA) CRASSIPINULUM. 
337 
The rays of the microhexactines (Plate 92 , figs. 9-15) are usually nearly 
equal and all quite straight or nearly so. Only rarely microhexactines are 
found in which one or two of the rays are distinctly curved in their middle-part. 
The rays are 3.5-7 n thick at the base, conical, pointed, and covered with spines. 
The spines on the proximal half of the ray are sparse, vertical or slightly inclined 
towards the centre of the spicule, and up to 2 n long. The spines on the distal 
half are more numerous, smaller, and rather strongly inclined towards the centre 
of the spicule. Most of the microhexactines have rather long and slender rays. 
These spicules (Plate 92 , figs. 9, 10) are 90-220 n in total diameter, and the 
basal thickness of their rays (3. 5-6. 5 m) is fairly in proportion to their size. 
Some microhexactines have much shorter and relatively much stouter rays. 
These spicules (Plate 92 , fig. 11) are only 65-80 ^ in diameter, and have rays 
as much as 7 /j. thick at the base. 
The rare monadine microhexadine-derivates appear as strongly spined tylo- 
styles. They are about 130 /x long, and 8 n thick near the tyle. The terminal 
tyle itself is about 9 m in diameter. 
Morphologically four kinds of amphidiscs can be distinguished: — 1, large 
amphidiscs with fairly smooth shaft and broad and short anchors, about a 
third of the whole spicule in length; 2, medium amphidiscs with a stout smooth 
shaft and broad and long anchors, usually a little more than half the whole 
spicule in length; 3, medium amphidiscs with a slightly spined, rather slender 
shaft, and long, narrow anchors, more than a third of the whole spicule in length; 
and 4, small amphidiscs with slender, spined shaft and rather short anchors, 
only about a third of the whole spicule in length. 
The amphidiscs belonging to the first kind are 375-480 n long, those belong- 
ing to the second kind 110-200 /x, those belonging to the third kind 112-137 /x, 
and those belonging to the fourth kind 31-106 /x. The first and the fourth kinds 
are accordingly distinguished both morphologically and biometrically. The 
second and third kinds, although distinguished in the same manner from the 
first and fourth, are distinguished from each other morphologically only, and 
not biometrically. 
As the measurements given above and the adjoined graph show, the gap 
in the length frequency-curve separating the fourth from the second and third 
kinds is much narrower than that separating the second and third from the 
first kind. In spite of the width of this gap, and the entire absence of transi- 
tions between the second and third kinds of amphidiscs on the one hand and the 
first kind of amphidiscs on the other, I am inclined to combine the first, second, 
