50 Mr. W. J. Sollas on a new 



form as the end of the series. It would not do, however, to 

 regard such a globular spicule as in any way resembling the 

 balls of the Geodidas ; for in the latter the growth is purely 

 radiate, in the former as purely concentric ; one is an over- 

 grown stellate spicule, the other an aborted acuate ; and the 

 difference between them is made very manifest by treatment 

 with boiling alkali, the Geodia-g\ohu\e dissolving rapidly 

 from the centre outwards, the Plocamia-cylm&ev obdurately 

 resisting all solution and never admitting the potash to its 

 interior. 



Finally, the acuates sometimes become bent upon them- 

 selves, to the extent of 90° or much more (PI. VII. fig. 18) ; 

 and as bending usually foreshadows branching, so we some- 

 times find a spicule putting forth a spine-like branch near its 

 extremity, into which enters correspondingly a branch from 

 the axial canal ; such a bifurcation of the end of one of these 

 acuates carries us much nearer the trifid spicules of the 

 Pachastrellidse and Pachytragidaa than the bi- and trifurca- 

 tion in the echinating spicules of Pleetiwnella, since in the 

 latter it is the head end which is so divided, while here, as 

 in the groups just mentioned, it is the pointed end of the 

 spicule. 



In conclusion we have to consider the behaviour of the 

 spicules when treated with boiling solution of potash. The 

 spined acuates under these circumstances readily dissolved 

 both within and without, in just the same manner as uniaxial 

 spicules usually do ; and I was able to make quite certain 

 that no branches were supplied to the spines by the axial 

 canal ; the spines are merely local thickenings of the exterior 

 of the spicule, not aborted branches as is the case with Plec- 

 tronella. 



Fig. 1. 



Echinating acuate after treatment with boiling potash (x 435). 



The dumbbell spicules and the large acuates with both ends 

 rounded seldom underwent internal solution, unless they had 

 been broken across so as to make the axial canal accessible. 

 Solution of the exterior proceeded rapidly, coat after coat of 

 the acerates being removed, and, indeed, of the dumbbell forms 

 as well, exposing the successive forms through which they 

 had passed during their growth ; and it is worth mentioning 



