306 HYALONEMA (PRIONEMA) FIMBRIATUM. 



2 Ai broad; proximally they become considerably narrower. Secondary inci- 

 sions subdivide the marginal parts of these primary into secondary lobes, so 

 that they attain a somewhat dendritic appearance (Plate 61, figs. 1-3). 



I am not aware that such fimbriate marginal membranes have hitherto 

 been observed in the amphidiscs of the Hexactinellida Amphidiscophora. They 

 may be compared with the thin marginal parts of the cladomes of the phyllo- 

 and discotriaenes of certain lithistid Tetraxonida. 



The large fimbriate amphidiscs (Plate 60, figs. 7-11) are 122-185 m long, 

 most frequently about 163 n. Their shafts are 2-4 n thick. Their thickness 

 is fairly proportional to the length of the spicule. The central tyle is 3-7 ju 

 in diameter, two thirds to twice as thick as the adjacent parts of the shaft. 

 The spines arising from the tyle are similar to those of the largest fimbriate 

 amphidiscs but proportionately smaller and usually not all curved in the same 

 direction (Plate 60, figs. 8, 10). The remainder of the shaft usually is more 

 spiny than in the largest fimbriate amphidiscs. The anchors are 39-80 /j. long ; 

 their maximum breadth is 26-55, their end-breadth 20-50 m- The difference 

 between maximum and end-breadth is 3-6 m, on an average (of ten measure- 

 ments) 5.2 fx. The proportion of the length to the maximum breadth of the 

 anchors is 100 to 50-73, on an average (of ten calculated proportions) 100 : 58.8. 

 A correlation between amphidisc-length and relative anchor-breadth is not 

 discernible in the amphidiscs belonging to this subgroup. 



The small fimbriate amphidiscs (Plate 60, figs. 12-14) are 64-99 /u long, most 

 frequently about 77 n. The shaft is 1.5-1.8 n, the central tyle usually 2-3 yu 

 thick. Strongly bent spines usually arise from the latter. Smaller short 

 and broad spines cover the remaining parts of the shaft quite densely. The 

 anchors are 23-36 m long and have a maximum breadth of 13-21 yu. The end- 

 breadth is usually 1-2 ^ less than the maximum breadth. The proportion of 

 length to maximum breadth of the anchors is 100 to 52-72, on an average (of 

 six calculated proportions) 100 : 59. 



The smallest fimbriate amphidiscs (Plate 60, fig. 15; Plate 62, fig. 15) are 

 36-54 yu long, most frequently about 52 and 38 m long. The shaft is 1.2-1.8 m 

 thick. A central thickening, as much as 2.8 /n in diameter, is sometimes dis- 

 cernible. Often, however, there is hardly any trace of such a tyle. The 

 shaft is covered with spines about 0.5 fi long. The anchors are 16-24 ^ long, 

 and have a maximum breadth of 9.5-13 /j. The end-breadth is 1-2 m less than 

 the maximum breadth. The proportion of the length to the maximum breadth 

 of the anchors is 100 to 55-75, on an average (of eight calculated proportions) 

 100 : 63.1. 



