30 



SIDONOPS ANGULATA. 



good deal longer than those of the plagio- and ortho-triaenes of the other two 

 varieties. The clades of the same cladome are equal (Plate 13, figs. 9, 11, 15, 18), 

 or sUghtly (Plate 13, figs. 5, 6, 8, 10, 13, 14, 19) or considerably (Plate 13, figs. 

 7, 12, 16) unequal in length. Generally the clades extend in a longitudinal plane 

 passing through the axis of the rhabdome; sometimes, however, (Plate 13, fig. 4) 

 an angular bend in a transverse plane is observed. vShort clades are often 

 (Plate 13, fig. 7, 12), long ones rarely (Plate 13, figs. 9, 12), nearly straight. 

 Generally the clades are markedly curved, concave to the rhabdome. This 

 curvature increases towards the ends of the clades. In some (Plate 13, figs. 3, 

 13-16, 19) the degree of this increment of curvature is slight, in others (Plate 13, 

 figs. 5-8) it is considerable. Sometimes the distal part of the clade is bent down 

 abruptly (angularly) (Plate 13, figs. 5, 10). The angles enclosed between the 

 clade-chords and the axis of the rhabdome are in the two specimens of var. 

 megana on an average 101.3 and 103°, in var. microana 105.6°, and in var. 

 orthotriaena 93.7°. These spicules are accordingly in the first two plagio- 

 triaenes, in the last orthotriaenes. The clade-angles of the three clades of the 

 same rhabdome are usually about equal, rarely (Plate 13, fig. 6) considerably 

 different. 



Very rarely one of the clades becomes quite rudimentary. Such diaene 

 spicules were found only in var. orthotriaena. 



DIMENSIONS OF NORMAL PLAGIO- AND ORTHOTRIAENES OF SIDONOPS ANGULATA. 





Length of 

 rhabdome 



mm. 



Thickness 

 of 



rhabdome 



at the 

 cladome 



1* 



Length of 

 clade-chords 



Angle between clade-chords 

 and axis of rhabdome 

 o 



limits 



average 



Var. megana, lobose specimen 



1.8-2.8 



50-82 



350-700 



92-104 



101.3 



Var. megana, massive specimen 



1.6-2.6 



50-80 



330-650 



89-111 



103 



Var. microana 



1.5-2.6 



50-77 



380-580 



91-112 



105.6 



Var. orthotriaena 



2-2.35 



47-78 



380-500 



91-98 



93.7 



The rhabdomes of the anaclades (Plate 12, figs. 1-15; Plate 13, fig. 17c) 

 are for a great part of their length nearly cylindrical. Of the long ones none 

 were found intact in the spicule-preparations, all being broken. The longest 

 fragment measured was 9 mm., the longest rhabdome observed intact was 6 mm. 

 in length. The rhabdomes of the anaclades of var. microana appear to be 



