20 
Mr. W. J. Sollas on a neio Genus and 
rays. Normally the paired rays are pointed, though somewhat 
abruptly, so as to resemble in optical section the point of a 
lancet, while the third ray may be either rounded off at the end 
or prolonged into a conical point, terminating one way just as 
often as the other. In the exceptional form, fig. 23, one of 
the smooth rays is rounded off at the end hemispherically, the 
other retaining its pointed termination; in fig. 25 both paired 
rays are terminated hemispherically. These last two varia¬ 
tions are of some importance, as showing the possibility of 
acerate spicules passing into aeuate ones : in a sponge con¬ 
taining only uniaxial acerate spicules, for instance, an aeuate 
variety, if it occurred, might be taken for an intruder from some 
other sponge; but with such a spicule as fig. 25 such an ex¬ 
planation is obviously impossible. 
In figs. 28, 29, & 32 we have varieties with four rays, a third 
smooth one having put in an appearance. These are true 
quadriradiate spicules, and thus seem to lead on to theTetrac- 
tinellid type. It must be observed, however, that they differ 
in one important particular from the quadriradiate pronged 
spicules of such sponges as Geodia } since in the latter the 
single bud produced from the spicule-cell grows inwards 
towards the centre of the sponge, while the three buds which 
form the prongs grow outwards away from the centre—just 
the very reverse of what occurs in Plectronella , where the three 
rays grow inwards towards the axis of the stem or papilla (on 
whichever the spicule is seated), and the fourth grows outwards 
away from the axis. An intermediate case is seen in Dercitus 
Bucklandi , where the four rays of its quadriradiate spicules 
are equal as regards length, and indifferent as regards direc¬ 
tion of growth. 
The quadriradiate spicules of Plectronella remind one also 
of the quinqueradiate spinicruces which echinate the spicular 
fibre of Meyerina , Hyalonema , and some other Hexactinellids. 
As regards the acerate spicules of our sponge, they likewise 
are very apt to sport into new varieties ; bifurcation of the 
spicule with an accompanying bifui-cation of its canal is not 
uncommon ; and spines occasionally sprout from the sides. 
Of the flesh-spicules one need not say much here; they appear 
to result from the fibrillation of the contents of a spicule-cell, 
and to be set free from it in some manner unknown ; they are 
of such tenuity that under a magnification of 1000 diameters 
they appear no thicker than pencil-lines ; and one need not add 
that they exhibit no axial canal. 
On passing in review the numerous departures from the 
normal spicule-forms figured in Pis. V. and VI. one becomes 
able to formulate the following propositions :— 
(1) The group of spicules comprising the bifid form and all 
