HYALONEMA (HYALONEMA) AGASSIZI. 
189 
Most of the other stalk-spicules exhibit, in their lower portion, the same 
transverse lines as the one described above, and in six of them the same spiral 
rods, combined to form tyle-like lamellae, are visible on portions of the surface 
near the end. 
The transverse lines may be considered as fissures in the superficial silica- 
layer. In the six spicules where it was observed, (and probably also in the 
others) the portions showing the superposed rows of spiral rods indicate that 
there are one or more silica-layers (composed of thin, spirally extending 
rods) quite different in structure from the rest. These layers are rendered 
visible where the disintegration (solution) of the spicule (which proceeds 
from the surface downwards) has just reached them; and their structure is 
probably brought out so clearly by the silica joining the rods having been partly 
dissolved. 
Being composed of layers differing in structure, one or more of which consist 
of superimposed rows of spirally arranged rods or threads, the stalk-spicules 
may, in respect to their internal structure, be compared to cables. 
No traces of backwardly directed spines or of terminal anchors could be 
found in any of the spicules. 
The amphidiscs (Plate 44 , fig. 17d; Plate 45 , figs. 40-G4; Plate 46 , figs. 1-16; 
Plate 47 , figs. 1-13). The biological length frequency-curve of the amphidiscs 
of Hyalonema agassizi, form A, shows (Fig. 5), that, as regards the frequency of 
those of different length, these spicules fall, like those of Hyalonema obtusum, 
into four groups: — large macramphidiscs, small macramphidiscs, large micram- 
phidiscs, and small micramphidiscs. The second and third of these groups are, 
in respect to their length frequency, not as clearly distinguished from each other 
as from the first and fourth respectively. The parts of the curve pertaining to 
the large macramphidiscs and the small micramphidiscs each have two culmi- 
nations, a principal, and a secondary. The measurements and examination of 
the amphidiscs of various length of the three other forms show that these also 
fall into the four groups mentioned, and that, at least in two of them (B and C), 
the gap between the small macramphidiscs and large micramphidiscs is not so 
distinct as in the others. In the forms B and C these two kinds of amphidiscs, 
which can be readily distinguished by differences in their shape, slightly overlap 
in respect to their length. 
The large macramphidiscs of form A (Plate 46 , figs. 2-5, 9, 12, 13; Plate 47 , 
figs. 1, 2, 5, 6, 10) are 134-242 n long, most frequently about 200 //. The shaft 
is straight, cylindrical, 7-13.5 m thick, and thickened abruptly at some point 
