HYALONEMA (HYALONEMA) OBTUSUM. 
167 
microhexac tines. They consist of six fairly equal rays joined at right angles, 
and measure 52-80 n in total diameter. Their rays are cylindrical, of nearly 
uniform stoutness throughout, and rounded at the end. They are 26-42 p long, 
5-15 m thick, and generally quite smooth. Their basal part is straight, their 
end-part either straight (Plate 39 , fig. 7) or more or less curved (Plate 39 , figs. 
8-10). Axial threads, terminating however a considerable distance below their 
ends, can be easily made out in the rays of these spicules. 
Among the amphidiscs of var. gracilis two main groups can be distinguished 
morphologically: — large forms, the largest of which have broad and rather short 
terminal anchors and a stout, spiny shaft; and small forms the largest of which 
have long and very slender terminal anchors and a slender shaft with very small 
spines. In each of these main groups, which I name macramphidiscs and micr- 
amphidiscs respectively, two subgroups can be distinguished: — in the macram- 
phidiscs larger forms with relatively shorter, and smaller forms with relatively 
longer, terminal anchors; in the micramphidiscs larger forms with long and 
slender anchors, and smaller forms with shorter and broader anchors. 
The biological length frequency-curve of these amphidiscs exhibits (Fig. 4) 
a gap between the lengths 54.76 p and 66.26 n. The amphidiscs, to which the 
part of the curve to the right of the gap pertains, are the amphidiscs referred to 
above as macramphidiscs; those to the left of the gap as micramphidiscs. Each 
of these two parts of the curve exhibits a conspicuous depression dividing it into 
two distinct elevations. These elevations correspond to the smaller and larger 
kinds of the macramphidiscs and the micramphidiscs, which are, as above stated, 
also distinguished from each other morphologically by the shape of their terminal 
anchors. 
Thus both the morphological and the biometrical qualities of these amphi- 
discs show that four kinds of these spicules are to be distinguished in var. gracilis: 
— large macramphidiscs, small macramphidiscs, large micramphidiscs, and small 
micramphidiscs. 
The amphidiscs of var. rohusta also fall into these four groups. 
The large macramphidiscs of var. gracilis (Plate 37 , figs. 20-22) are some- 
what infrequent. They are 250-356 n long, most frequently about 1 264 /x, and 
have a straight shaft 8-14 n thick. This is thickened slightly and gradually 
to 14-22 /x at the ends, and abruptly in its middle-part to a central tyle 15-18 n 
in diameter. The tyle never appears to lie quite in the middle ; the difference 
1 This phrase “ most frequently about ” refers, throughout the descriptions, to the summit of that 
part of the length frequency-curve of the graph which pertains to the amphidiscs in question. 
