44 



SIDONOPS OXYASTRA. 



The plagiotriaenes (Plate 6, figs. 6-13; Plate 8, fig. 4b, c) have a conical 

 sharp-pointed rhabdome, which is straight or slightly curved in its acladomal 

 part. The rhabdome is 1-1.65 mm. long and, at the cladome 24-40 p- thick. 

 The clades are usually conical and pointed; rarely one (Plate 6, fig. 7), two, or 

 all three (Plate 6, fig. 6) are reduced in length and rounded off terminally. 

 The normal pointed clades of the same cladome are usually about equal in 

 length, more rarely distinctly unequal (Plate 6, figs. 8, 10). The basal part 

 of the clades is directed obliquely upward and always curved, concave to the 

 rhabdome, their distal part is directed outward and straight or slightly curved 

 in the opposite direction. The development of this upward bend of the distal 

 part of the clade is usually proportional to its length. The chords of the 

 normal (pointed) clades are 250-285 fi long and enclose angles of 100-118°, on an 

 average 108.5°, with the axis of the rhabdome. 



In a spicule-preparation I found a triaene with a clade-chord 350 long, 

 enclosing an angle of 90° with the rhabdome. Perhaps this orthotriaene is a 

 foreign spicule. 



The rhabdomes of the small dermal anaclades (Plate 6, figs. 15-18, 19b, 

 20b, 23) appear — I found none intact in the spicule-preparations — to be over 

 1 mm. long. They are, at the cladome, 5-12 // thick. Their cladomes are very 

 variable. The most frequent forms are anatriaenes (Plate 6, figs. 17, 18, 19b, 

 20b) with a protuberance on the apex of the cladome. Their clades are pointed, 

 very rarely blunt, more or less angularly bent, concave to the rhabdome and 

 often rather unequal. This inequahty is sometimes carried to the extent of a 

 complete suppression of one or two clades, whereby diaene and monaene forms 

 are produced. Not infrequently the apical protuberance is replaced by a long, 

 blunt (Plate 6, fig. 15, 16) or, more frequently, pointed epirhabd. Most of 

 these mesanaclades are quite regular mesanatriaenes (Plate 6, fig. 15). Some 

 of them are, however, rendered irregular by one of the clades extending upwards, 

 proclade-fashion (Plate 6, fig. 16). These mesanatriaenes were found only at 

 the place where the anaclades are abundant. The chords of the clades of the 

 more regular triaene and mesotriaene anaclades are 15-24 /< long and enclose 

 angles of 40-65°, on an average 57°, with the axis of the rhabdome. The 

 clades of the diaene and monaene anaclades are longer, some of them attain- 

 ing a length of 30 The epirhabd is, when fully developed, straight, conic, 

 sharp pointed, and 65-75 pt long. 



Although the different kinds of euasters are, to some extent, connected 

 by transitional forms, three categories can readily be distinguished: large 



