270 



ERYLUS OXYASTER. 



able. Sometimes two similar branches lie opposite each other in a straight 

 line (Plate i, fig. 17 the two lower ones, 19). The axial threads of the shaft and 

 the branches are either joined in a regular manner, or slight irregularities occur 

 at their junction. The most remarkable of these were observed in the two spic- 

 ules represented on Plate 4, figs. 15 and 18. The axial thread of the single 

 branch of the former and that of the lowest one of the latter do not reach down 

 to the axial thread of the shaft, which passes the junction unaltered, but termi- 

 nate with a bulbous thickening at a distance of about 3 /( from it. 



The microrhahds (Plate 3, figs. 29-31, 32a, 35a; Plate 4, figs. 28-33a) are 

 more or less curved, centrotyle amphistrongyles, and generally isoactine. The 

 curvature is either uniform or one or both ends are also abruptly bent in the 

 direction of the curvature. The isoactine microrhabds are usually 31-47 fi long, 

 but occasionally very much larger ones, up to 93 fi in length, are observed (Plate 

 4, fig. 31). The ordinary microrhabds are, near the centre (tyle), 3.5-4.5 /jl 

 thick; the tyle measures 4.5-5.5 ft, usually about 1 fi more than the adjacent 

 })arts of the spicule, in diameter. Towards the rounded ends the actines taper 

 gradually to about 2 fi. The rare giant microrhabds are thicker in proportion 

 to their greater length. The centre of the spherical tyle usually lies in the axis 

 of the spicule ; sometimes, however, it is eccentric and then the tyle bulges much 

 more on one side than on the others. In some microrhabds one actine is reduced 

 in length; these appear as anisoactines. In a few one of the actines is completely 

 suppressed ; these appear as blunt tylostyles. The anisoactine microrhabds are 

 shorter and also somewhat thicker than the ordinary isoactine ones. 



The rare plagiotriaenes (Plate 4, fig. 20) have a straight, conical rhabdome, 

 about 0.9 mm. long and, at the cladomal end, 75-90 /t thick. The clades are 

 nearly straight, about 0.7 mm. long, and enclose angles of 109-112° with the 

 rhabdome. The breadth of the cladome is 1.3-1.4 mm. 



The regular dichotriaenes (Plate 4, figs. 1-5, 21-23) have a fairly straight, 

 conical rhabdome, 0.6-1.6 mm. long and, at the cladomal end, 70-105 fi thick. 

 The main clades are straight, 250 400 fj. long and enclose angles of 109-120° 

 with the rhabdome. The end clades are conical, pointed, and straight or, more 

 rarely, slightly curved inwards (Plate 4, fig. 5, below), and 50-450 fx long. The 

 cladome is 0.9-1.5 mm. broad. 



In the rare irregular dichotriaene-derivates either the clades are reduced in 

 number or the rhabdome reduced in length, or both. Forms with two and 

 with only one clade (dichodiaenes and dichomonaenes) have been observed. 

 The reduced rhabdomes are cylindrical, rounded at the acladomal end, 200- 



