— 43 — 

 Tisiphonia? sp. 



(Plate UI, 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 ol 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. 



