SIDONOPS BICOLOR. 



51 



slightly curved, either simply, concave towards the rhabdome (Plate 9, fig. 2), 

 or in the shape of an S, in such a manner that the proximal part is concave, the 

 distal part convex towards the rhabdome (Plate 9, fig. 13). The clades are 

 280-700 fi long; their chords enclose angles of 103-122° with the rhabdome. 



DIMENSIONS OF THE NORMAL PLAGIOTRIAENES. 



The Im-ge choanosomal oxyasters (Plate 10, figs. 6-13a; Plate 11, figs. 6b, 

 8b, 9) have from one to twelve rays. Forms with one ray are exceedingly rare 

 and were observed only in a specimen from Station 4420. Forms with two rays 

 were not found. Three-rayed forms are met with in small numbers in the 

 specimens from Stations 3168 and 4551, four-rayed ones also in the specimens 

 from Station 4531. By far the most frequent forms are those with from five 

 to nine rays which occur in large numbers in all the specimens. The many- 

 rayed oxyasters pass into the oxysphaerasters. The rays are 1-2.8 fi thick at 

 the base, conical and straight. In the many-rayed oxyasters they are always, 

 in the few-rayed ones usually, sharp jjointed. In the few-rayed oxyasters the 

 rays are irregularly distributed and apparently not always quite concentric, 

 many of these spicules appearing somewhat metastrose. In the oxyasters with 

 five or more rays, the rays are quite regularly distributed and concentric, but 

 these asters are also occasionally rendered somewhat irregular by one of 

 their rays being bifid. The distal parts of the rays are always spiny. In most 

 cases the spines are confined to the distal third of the rays, in some they cover 

 as much as the distal two thirds. The spines are either numerous and small, 

 or sparse and large. The sparse, large spines are slender and rise vertically 

 from the ray. A central thickening 2.8-6 a* in diameter is nearly always devel- 

 oped. The whole oxyaster measures 20-34 n in diameter. The length and 



