HYALONEMA (HYALONEMA) 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 cylindroconic, truncate, 

 or rarely irregular; usually they are irregularly curved, some in one direction, 

 others in others. These spines reach 5 ^u in length and 2 n 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 curvetl 

 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 curvatm-e of the teeth is such 

 that their end-parts generally shghtly converge towards the shaft. Sometimes, 

 however, they are parallel to it, or even shghtly 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 55-286 n long, in 

 form A 105-197 p. 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 micramphidiscs with slender anchors are not numerous. Those of 

 form B (Plate 63, fig. 36) are 30.5-48 ^ long, most frequently about 38. The 

 shaft is usually a Uttle 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 /x long, usually a third to two fifths of the 



