280 



ERYLUS SOLLASII. 



clades has been observed only in race I, form D, and in race III. It affects 

 either both end clades of a pair in a similar manner (Plate 3, the left ones in fig. 4) 

 or one of them only (Plate 1, the upper one in fig. 10; Plate 3, the upper in fig. 2 

 and the right ones in figs. 2-5). The proportion of the length of the main clade 

 to the length of the end clades is, as a comparison of figs. 6 and 9 on Plate 1 

 shows, subject to very considerable variation. The main clades are 70-270, the 

 end clades 10-160 [i long. The cladomes are 160-550 p. broad. The triaenes 

 of the small specimens of race I, form A, have the narrowest cladomes (average 

 of the three largest 373 p), those of the large specimen of race III, the broadest 

 (average of the three largest 530 /<). The angle enclosed by the axis of the 

 rhabdome and the chords of the simple clades and the stems (main clades) of 

 the branched ones, is 86-116° (general average 103.4°). It is smallest in the 

 triaenes of race I, form A (average 97°), and largest in those of races II and III 

 (average 107 and 107.5°). Thus most of these spicules are plagioclades, some 

 orthoclades. (See table, p. 281.) 



In race II some spicules, 310-330 /i in diameter, composed of two simple 

 (Plate 1, fig. 13) or branched (Plate 1, the right one in fig. 14) clade-like, and 

 two short, conical, blunt rudimentary rays, have been observed. These spicules 

 appear to be derivates of the triaenes described above. 



The acanthtylasters (Plate 1, figs. 36-40c; Plate 2, figs. 1-4, 8-11; Plate 3, 

 figs. 7, 8, 25, 26c) are destitute of a central thickening and have from two to 

 fourteen, a few perhaps more than fourteen, rays. The rays are concentric and 

 nearly always uniformly distributed, simple, and equal in size. Acanthtylasters 

 with rays unequally long or branched (bifurcate) have been only very rarely 

 observed. The rays of the larger acanthtylasters (Plate 2, figs. 1, 2, 8, 9; Plate 

 3, figs. 7, 8) are cylindroconical and at the base 1-3 usually 1.5-2.3 ft thick. 

 They taper distinctly towards the end, and are, at their thinnest point a short 

 distance below the end, 0.7-1.5 fi thick. The rays of the smaller ones (Plate 2, 

 figs. 10, 11) are somewhat more cylindrical and only 0.4-1.5 p. thick. The 

 rays invariably bear spines, some of which always congregate at the end of the 

 ray and here form a terminal, acanthtyl cluster 1.3-3.5 p in diameter. The size 

 of the spines is on the whole proportional to the size of the aster. Apart from 

 this they are subject to considerable variation. Sometimes numerous small 

 and insignificant (Plate 3, fig. 7), sometimes numerous medium sized (Plate 2, 

 figs. 8, 9), and sometimes only one or a few very large spines, 1-1.6 p in length 

 (Plate 2, figs. 1,2; Plate 3, fig. 8), arise from the sides of the rays. In the two- 

 rayed acanthtylasters a cluster of spines arises from the centre of the spicule. 



