MATERIALS FOR A MONOGRAPH OF THE ASCONS. 315 
III. Tae Trrrapiate Systems. 
One of the distinctive characters of the genus Leuco- 
solenia, as defined by me in a former memoir (1905 [2]), is 
seen in the form of the triradiate systems. In Clathrina 
these spicules are typically equiangular, and only very 
exceptionally depart from this type, equiangular triradiates 
being in all cases present and constituting the principal 
skeleton. In Leucosolenia, on the other hand, the triradi- 
ates exhibit typically a bilaterally symmetrical pattern; they 
have a unpaired straight ray and two paired, usually curved 
rays, and corresponding to these differences in the rays, there 
is an unpaired angle greater than 120°, and two paired angles 
each less than 120°. Only exceptionally, and as one of many 
variations, are the three rays equally developed, or the three 
angles each indistinguishable from 120°. The two species in 
which I have studied the spicule-formation are further dis- 
tinguished from one another by the characters of their tri- 
radiates. In Leucosolenia complicata the unpaired ray 
of the triradiate is almost invariably longer than the paired 
rays; in L. variabilis the unpaired ray is constantly the 
shorter of the three. Moreover, in L. variabilis the 
unpaired angle is much more obtuse than in L. complicata, 
and often approaches 180°, while in L. complicata it more 
nearly approximates to 120°. 
If an ordinary museum-specimen, preserved in spirit, of 
L. complicata be taken, and a piece of the thin body-wall 
cut out, mounted without further treatment, and examined 
microscopically, with moderately high magnification, it is 
easy to see many stages of the growth of the triradiates, 
especially if the preparation be examined by polarized light 
with crossed prisms, when the spicules stand out brilliantly 
illuminated on a dark background. It is then seen that the 
stages of growth are as follows :—first a small piece of calcite 
in the form of an isosceles triangle, in which the base is 
about one-third the length of the sides; this represents the 
