Tisiphonia? sp. 
(Plate III, figs. 16—23). 
Compound trifid spicules, with stout, straight, more or 
less developed shaft and very robust head rays, which are 
bifurcated and widely extended in a horizontal direction. As 
will be seen by the figures, there is great variation in the 
relative dimensions of the arms and the shaft in these spicules. 
In some specimens (fig. 17) the shaft is comparatively long 
and the head rays stout, and but moderately expanded. The 
example figured has a length of 2,115 mm., the shaft is 
0,112 mm. in diameter, and the extension across the head 
1,26 mm. Another spicule (fig. 18) has a shorter and thicker 
shaft and also stouter head rays. Figs. 16 and 20 show 
forms in which the head rays are slender aud more divergent. 
In fig. 21 the shaft is truncated and rounded at the end. In 
fig. 22 the shaft resembles a stout cone and the head rays 
are also very robust and obtusely pointed. This specimen 
has a length of 1,35 mm.; thickness of shaft 0,247 mm. and 
the extension of the head 0,967 mm. Lastly in figs. 19 and 
20 we have examples in which the head is largely developed 
and the shaft ot the spicule has become reduced to little 
more than a mere rounded prominence. The width of the 
head in these specimens is 1,6 mm. The figures show also, 
how unequally the head rays of these spicules are developed ; 
in some examples the primary rays are symmetrical, compa- 
ratively long and extend some distance from the centre be- 
fore they bifurcate and then give off long pointed secondary 
rays (figs. 16, 20); in other specimens the primary rays 
are so wide that they amalgamate and form a solid laminated 
centre from which the unequally developed, obtusely blunted 
secondary rays are given off. In many specimens the canal 
system is very distinctly shown, those of the arms. radiating 
from the centre of ‘the shaft and sending a branch to the 
extremity of each ray. 
