232 HYALONEMA (HYALONEMA) TYLOSTYLUM. 



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 ix thick, usually about 2 ,u. The rare pentactine 

 forms differ from the hexactine ones only by having five rays instead of six. 



The amphidiscs are 29-410 n long. Their length frequency-curve exhibits 

 one great interruption between 49 and 116 /i. The amphidiscs under 49 ^ in 

 length (that is those between 29 and 49) have relatively shorter and broader 

 anchors, the amphidiscs over 116 m 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- 

 gmshed: — micramphidiscs 29-49 /z long with broad anchors, and macramphi- 

 discs 116^10 n long with slender anchors. The length frequency-curve of the 

 macramphidiscs is somewhat irregular, and exhibits a broad depression at about 

 250 fi. In the amphidiscs under 250 /x in length the average proportion of 

 anchor-length to anchor-breadth is 100 : 58.3, in those over 250 /x in length this 

 proportion is 100 : 73.7. The macramphidiscs can therefore be subdivided into 

 two groups: — small macramphidiscs 116-250 ^ long with relatively more 

 slender anchors, and large macramphidiscs 250-410 m 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 macramphidiscs (Plate 69, figs. 14, 19, 24, 25) are 260-410 fx, most 

 frequently about 378 ^ long. The shaft is straight, for the greater part of its 

 length cylindrical, and 6-12 ^ 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 fi thicker than the cylin- 

 di'ical part of the shaft. The central tyle is 13-28 ^ in transverse diameter, that 

 is, 6-18 M 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 thi'ough 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 /x long and 3-6 m thick. The 



