HYALONEMA (HYALONEMA) OBTUSUM. 167 



microhexactines. They consist of six fairly equal rays joined at right angles, 

 and measure 52-SO n in total diameter. Their rays are cylindrical, of nearly 

 uniform stoutness throughout, and rounded at the end. They are 26-42 fi long, 

 5-15 n 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 m and 66.26 ai. The amphidiscs, to which the 

 part of the cur\-e to the right of the gap pertains, are the amphidiscs referred to 

 abo\'e 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 micramj^hidiscs, 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 /our kinds of these spicules are to be distinguished in var. gracilis : 

 — large macramphidiscs, small macramphidiscs, large micramphidiscs, and small 

 micramphidiscs. 



The amphidiscs of ^•ar. robusta 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 ^ long, most frequently about ' 264 n, and 

 have a straight shaft 8-14 ii thick. This is thickened slightly and gradually 

 to 14-22 fi at the ends, and abruptly in its middle-part to a central tyle 15-18 m 

 in diameter. The tyle never appears to lie quite in the middle ; the difference 



'This phra-se " 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. 



