226 HYALONEMA (HYALONEMA) TENUIFUSUM. 
In specimen (b) I found a microhexactine with a branch-ray on one of its 
rays. 
In the irregular microhexactines (Plate 67, fig. 8; Plate 68, figs. 7-9, 12-15) 
two opposite rays, lying in the same spicular axis, are long and well-developed, 
the other four variously reduced. The two long rays may be considered as 
apical, the other four as lateral rays. The two long apical rays are considerably 
longer than the rays of the regular microhexactines, so that the maximum diam- 
eter, that is the total length of these spicules, exceeds the diameter of the regular 
microhexactines. The apical rays are straight or only very slightly curved in 
their basal and_middle-part, conical, pointed, smooth, or slightly roughened by 
very minute spines, and are 1—2.2 » thick at the base. The degree of reduction 
of the four lateral rays is equal or unequal, and is generally very considerable. 
They may all be present and equally long, or one, two, or three of them may 
be shorter or altogether absent. When one or more of these rays have dis- 
appeared altogether, the remaining lateral rays are usually very short. Spic- 
ules in which three of the lateral rays have disappeared altogether whilst the 
fourth is only slightly reduced in length (Plate 68, fig. 13) are very rare. It is 
to be noted that the reduced lateral rays are not only shorter, but often also 
thinner than the apical, the difference in the basal thickness of the apicals and 
laterals often amounting to 0.5 pu. 
The irregular microhexactines in both specimens are 125-400 » long and 
8-112 u broad. There is a very clearly pronounced correlation between the 
length of the apical rays (the total length of the spicule) and the degree of 
reduction of the lateral rays (the total breadth of the spicule); the longer the 
apicals and the whole spicule, the shorter are the laterals and the narrower is the 
whole spicule. 
The irregular microhexactines 125-170 long are 72-112 broad. 
“ 190-220 “ “ 33-85 fa 
es a 280-400 ‘“ “ 8-14 ¢ 
Thus these irregular microhexactines form a series connecting the (shorter) 
regular microhexactines described above with the (longer) minute centrotyle 
amphioxes described below. 
The minute centrotyle amphioxes (Plate 67, fig. 12; Plate 68, figs. 16, 17) 
are more or less curved, the central part usually in one direction, the two end- 
parts in the opposite, so that these spicules generally look like bows. They are 
mostly 580-830 uw long, and 1.5-2 » thick near the middle. The central tyle is 
oval and measures 3-5 u in transverse diameter. The two rays are conical and 
