HYALONEMA (HYALQNEMA) PLACUNA. 
217 
The three kinds of mesamphidiscs (Plate 63 , figs. 29-35; Plate 65 , fig. 2) 
all have much the same shape. They are distinguished morphologically by the 
anchors of the larger being on the whole somewhat narrower than the anchors 
of the smaller, and biometrically by well-marked depressions in the length 
frequency-curve. The* shaft is gradually thickened towards the ends and 
abruptly thickened in or near the middle to a central tyle. The latter bears a 
verticil of conspicuous spines which are cylindrical or cylinclroconic, truncate, 
or rarely irregular; usually they are irregularly curved, some in one direction, 
others in others. These spines reach 5 m in length and 2 ^ in thickness. In the 
large and medium mesamphidiscs the remainder of the shaft bears remarkably 
numerous similar spines; these are shorter and usually straight. The terminal 
anchors are narrow. The teeth arise nearly vertically from the shaft, are curved 
very strongly in their basal part and much less strongly and quite uniformly in 
their distal and middle-part. Often they are abruptly bent down near the base. 
Another inconsiderable abrupt inward bend is frequently seen a short distance 
from the end. When such a bend is present the distal part of the tooth, lying 
beyond it, is generally straight or slightly curved in the opposite direction, convex 
to the shaft (Plate 63 , figs. 29-32). Altogether the curvature of the teeth is such 
that their end-parts generally slightly converge towards the shaft. Sometimes, 
however, they are parallel to it, or even slightly divergent. 
The measurements of the three kinds of mesamphidiscs of the two forms 
are tabulated on p. 218. 
This table shows that the mesamphidiscs are in form B 55-280 m long, in 
form A 105-197 In the single large spicule of this kind observed in form A 
the anchors are broader (proportion of anchor-length to anchor-breadth 100 : 73) 
than in the others (proportion of anchor-length to anchor-breadth 100 to 42- 
66). On the whole, as stated above, the smaller these spicules the narrower 
the anchors; the average proportion of anchor-length to anchor-breadth being 
in form B : — 
in the large mesamphidiscs 100 : 58.9 ; 
in the medium “ 100 : 54.7 ; 
and in the small “ 100:52.7. 
The large micrampkidiscs with slender anchors are not numerous. Those of 
form B (Plate 63 , fig. 36) are 30.5-48 /x long, most frequently about 38. The 
shaft is usually a little under 1 ^ thick and slightly thickened at some place near 
the middle. It usually bears several minute low and broad spines in the middle- 
part. The terminal anchors are 14-20 n long, usually a third to two fifths of the 
