HYAL0NE:\IA (HYALOXEMA) OBTUSUM. 109 



The two anchors of the same spicule are equal or slightly unequal in size. 

 The greatest differences between them in length and breadth observed were 12 

 and 8 yu respectively. The anchors are 70-100 n, that is generally a little less 

 than a third of the whole spicule in length, and 70-95 tx broad. The proportion 

 of length to breadth in these anchors is 100 to 60-108, on an average 100 : 93.2. 



Each anchor consists of eight recurved teeth. The teeth of the same anchor 

 are fairly equal and regularly arranged. Their axes extend in planes passing 

 through the axis of the shaft and enclosing angles of 45° with their neighbours. 

 The individual teeth arise nearly vertically from the shaft, and then curve con- 

 cave towards it. This curvature in the basal part of the tooth is for some 

 distance fairly uniform, but it decreases distally. Towards its end the tooth is 

 hardly at all concave to the shaft, or straight, or even slightly convex to the 

 shaft. The whole curvature amounts to about 90°, the end-parts of the teeth 

 being nearly parallel. Seen en face (from above) the individual teeth are elon- 

 gate oval in appearance, 10-16 /i broad in the middle, and rounded or pointed at 

 the end. Seen in profile they appear stoutest at the base and are at first gradu- 

 ally, and near the end abruptly, attenuated to a sharp point. Each anchor- 

 tooth somewhat resembles a T-iron. It consists of an outer band-shaped part, 

 and an inner keel. The outer band-shaped part is broadest in the middle, at- 

 tenuated towards both ends, and bent, concave to the shaft, both transversely 

 and longitudinally. Its transverse convexity increases, and its longitudinal 

 convexity decreases, towards the distal end. The inner keel is highest at the 

 base of the tooth; towards its distal end it becomes lower, at first gradually, 

 then more abruptly. 



The large macramphidiscs of var. robusta (Plate 40, figs. 1, 2, 19) are similar 

 to those of var. gracilis, but larger and provided with somewhat thicker shafts 

 and broader anchors. They are 235-335 m long, most frequently about 297 fi. 

 The shaft is 10-17 m thick. Its central tyle measures 15-18 m in diameter. The 

 anchors are 70-100 ^ in length, that is about a third of the whole spicule, and 

 90-110 fj. broad. The proportion of the length to the breadth of these anchors 

 is 100 to 95-150, on an average 100 : 117.6. 



The S7nall macramphidiscs of var. gracilis (Plate 34, fig. 19b; Plate 35, 

 fig. 24b; Plate 37, figs. 12-19; Plate 38, figs. 4-8) are 86-212 m long, most fre- 

 quently about 164 n. The shaft is 2.5-9 fi thick, and thickened in or near the 

 middle to a central tyle 4-12 m in diameter. The shaft bears spines similar to 

 those on the shafts of the large macramphidiscs. These spines are 5-12 ^ long 

 and 2-3.5 /x thick. The terminal anchors are 32-72 n in length, usually a little 



