304 HYALONEMA (PRIONEMA) FIMBRIATUM. 



Plate 62, fig. 27) form a biometrically homogeneous and well-defined group, 

 entirely separated dimensionally from the (larger) macramphidiscs on the one 

 hand and the (smaller) large fimbriate amphidiscs on the other. 



The largest fimbriated ampliidiscs are 200-323 ^ long, most frequently 

 about 250 fi long. The shaft is straight, on the whole cylindrical, and 4-7.5 n 

 thick. Its thickness is fairly proportional to the length of the spicule. The 

 shaft is thickened slightly and gradually towards the ends, and considerably 

 and abruptly somewhere in its middle-part. The latter thickening, the central 

 tyle, is usually some distance, occasionally (Plate 60, fig. 5) very far away from 

 the real (geometrical) centre of the shaft, and measures 8-14 n in transverse 

 diameter. The proportion between the thickness of the adjacent parts of the 

 shaft to the thickness of the tyle is 100 to 160-325. From the central tyle a 

 verticillate bunch of spines arises. These spines (Plate 62, fig. 27) are 5-15 ^ 

 long, at the base 2-2.8 m thick, cylindrical, and terminally simply rounded, or 

 attenuated distally and truncate with a cluster of exceedingly minute secondary 

 spinelets on the terminal face. The (primary) spines are curved. The curva- 

 ture is irregular, and a rather abrupt angular bend usually occurs somewhere 

 near the middle of the length of the spine. Generally all the spines of the bunch 

 are, as in the macramphidiscs, curved in the same direction longitudinally or 

 obliquely (spirally). The parts of the shaft outside the central tyle bear very 

 numerous spines, the largest of which are 1 ^ long, 1.5 yu broad, cylindrical, and 

 provided with a cluster of exceedingly minute secondary spinelets on their flat 

 or rounded terminal face (Plate 60, figs. 5, 6; Plate 62, fig. 27). The degree 

 of development of these scattered (primary) spines is very variable ; often they 

 are reduced to hardly perceptible protuberances on the surface of the shaft 

 (Plate 60, fig. 3). 



The two anchors of the same spicule are fairly equal, or rather unequal, in 

 size. They usually consist of eight teeth. The anchors are 72-136 m long, and 

 43-67 M broad at their broadest point and attenuated distally, their ends meas- 

 uring only 32-60 m in transverse diameter. The difference between the maxi- 

 mum and end-breadth of the anchors is 3-18, on an average (of 29 measure- 

 ments) 7.8 ;u. The proportion of the length to the maximum breadth of the 

 anchors is 100 to 42-72, on an average (of 29 calculated proportions) 100 : 52.2. 

 The largest (longest) anchors are on the whole narrower than the smaller (shorter) 

 ones. The average proportion of length to maximum breadth is in the anchors 

 over 120 m in length 100 : 47.8, in those under 100 n in length 100 : 55.7. 



The individual anchor-teeth arise nearly vertically from the end of the 



