560 DR. R. VON LENDENFELD ON THE SYSTEMATIC [Dec. 21, 
clature of Hexactinellid spicules is here, of course, accepted en bloc, 
and his terms will be defined below. Sollas (1453) has used a 
number of terms in his preliminary report which I do not under- 
stand and which presumably nobody else understands either. It is, 
therefore, much to be regretted that the greater part of them are 
unexplained. In consequence of this I will, for the present, abstain 
from attempting to compile a nomenclature of Tetraxonid spicules 
pending the publication of Sollas’s full report, in which, we may 
hope, he will explain his new terms. 
The spicules of Sponges are, as a rule, of such shape that they 
appear as more or less modified geometrical figures with definite axes. 
The axes are always represented by a non-skeletal rod (the so-called 
axial canal), round which the silica or lime is precipitated in con- 
centric layers. There may be one such axis, or there may be more 
than one. 
Hickel (627) drew attention to this crystalline regularity of 
sponge-spicules, which has been of great importance in studying the 
skeletal elements of Sponges. 
We can divide the sponge-spicules, as we do the Sponges, into the two 
groups Calcarea and Silicea, according to their chemical composition, 
Within each group we distinguish series of forms according to the 
number and position of the axes. ‘The validity of this classification 
is proved by the correlation of these different kinds of spicules with 
other organs in the Sponges. 
The following are the different kinds of spicules :— 
I. Group Sprcuta CaLcaRrEa. 
Composed chiefly of carbonate of lime. 
1. Monaxonia. 
With one straight or curved axis, rod-shaped. 
2. Triazxonia. 
With three distinct axes which may lie in one plane or not. 
When one of the rays of this tri-act spicule becomes rudimentary, 
Diaxonia can theoretically be produced. It is, however, advantageous 
to consider the Diaxon spicules as part of the Triaxonia. The 
calcareous triaxon spicules have only three rays—triact. 
3. Tetravonia. 
With four axes and four rays—tetract. The points form the 
corners of a triangular pyramid. Generally three axes, or rays, are 
equivalent (tangential), and one (radial) is differentiated, longer or ° 
shorter than the others. 
II. Group Sercuna Srxicea. 
Composed chiefly of silica. 
1. Anawonia. 
Without definite axes and with numerous rays—polyact. 
To this group belongs one kind of spicule only, namely the stellate 
and its derivatives. 
