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. 40-64; 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 obtusuni, 

 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 ha\'e 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 amjihidiscs, 

 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 ^ long, most frequently about 200 m- The shaft 

 is straight, cylindrical, 7-13.5// thick, and thickened abruptly at some point 



