232 
HYALONEMA (HYALONEMA) TYLQSTYLUM. 
ray or, rarely, several rays are markedly curved. In such rays the curvature 
is not confined to the end-part. The rays are conical, fine-pointed, distinctly 
spiny, and at the base 1.5-2. 7 p thick, usually about 2 p. The rare pentactine 
forms differ from the liexactine ones only by having five rays instead of six. 
The amphidiscs are 29-410 p long. Their length frequency-curve exhibits 
one great interruption between 49 and 116 p. The amphidiscs under 49 p in 
length (that is those between 29 and 49) have relatively shorter and broader 
anchors, the amphidiscs over 116 p in length (that is those between 116 and 410) 
have relatively longer and narrower anchors. Thus both from a morphological 
and a biometrical point of view, two kinds of amphidisc are to be distin- 
guished : — micramphidiscs 29-49 p long with broad anchors, and macramphi- 
discs 116-410 p long with slender anchors. The length frequency-curve of the 
macramphidiscs is somewhat irregular, and exhibits a broad depression at about 
250 p. In the amphidiscs under 250 p in length the average proportion of 
anchor-length to anchor-breadth is 100 : 58.3, in those over 250 p in length this 
proportion is 100 : 73.7. The macramphidiscs can therefore be subdivided into 
two groups: — small macramphidiscs 116-250 p long with relatively more 
slender anchors, and large macramphidiscs 250-410 p long with relatively 
broader anchors. The length frequency-curve of the micramphidiscs is quite 
regular and has only one very pronounced summit. These spicules form a 
single, homogeneous group. 
The large viacr amphidiscs (Plate 69 , figs. 14, 19, 24, 25) are 260-410 p, most 
frequently about 378 p long. The shaft is straight, for the greater part of its 
length cylindrical, and 6-12 p thick. It is slightly and gradually thickened 
towards its ends, and to a greater extent and much more abruptly thickened at 
or near the middle to a central tyle. The ends are 2-7 p thicker than the cylin- 
drical part of the shaft. The central tyle is 13-28 p in transverse diameter, that 
is, 6-18 p more than the adjacent parts of the shaft. It bears a verticil of spines 
which are cylindrical, or only very slightly distally attenuated, terminally simply 
rounded or more rarely truncate, and more or less, often very considerably, 
curved (Plate 69 , figs. 14, 19, 24, 25). The curvature is generally simple and 
extends in a plane which passes through the axis of the shaft. Usually all the 
spines of the tyle are curved in the same direction (towards the same end of the 
spicule) (Plate 69 , figs. 19, 24). Occasionally the majority of them are curved 
towards one end and a minority of one or two towards the opposite end (Plate 
69 , figs. 14, 25). Generally the spines are simple, exceptionally bifurcate (Plate 
69 , fig. 14, the left one). These spines are 7-17 p long and 3-6 p thick. The 
