SIDONOPS ANGULATA. 



31 



considerably shorter than those of the anaclades of the other two varieties. 

 The acladomal end of the rhabdome is attenuated and pointed, or cyhndrical 

 and terminally rounded (Plate 12, fig. 15). At the cladomal end the rhabdomes 

 are in var. megana 7-39 p., in the two other varieties 10-18 thick. There is 

 always on the summit of the cladome an apical protuberance which, however, 

 does not contain a prolongation of the axial thread of the rhabdome, the latter 

 terminating at the point where the axial threads of the clades arise from it. 

 The axial threads of the clades are directed obliquely downwards in their basal 

 portion (Plate 12, figs. 1, 5, 13, 14). After extending a short distance in this 

 direction they bend outward angularly and then follow the axes of the clades. 

 This peculiarity of the axial threads of the clades is doubtlessly the cause of 

 the formation of the apical protuberance of the cladome. Most of the anaclades 

 have three fairly equal clades (Plate 12, figs. 1-6, 8, 9, 12-14). In some there 

 are two longer and one short clade (Plate 12, fig. 7). In not a few of the 

 anaclades of var. microana one clade has disappeared entirely, so that these 

 spicules are diaenes (Plate 12, figs. 10, 11). The clades are conical, pointed, and 

 when long, distinctly curved, concave towards the rhabdome (Plate 12, figs. 1, 

 3, 5-10). In short clades (Plate 12, figs. 2, 4, 11-14) this curvature is usually 

 slighter, often hardly perceptible. The distal part of the clade is usually curved 

 less than the basal. Sometimes (Plate 12, figs. 3, 5, 6) an abrupt angular bend 

 is observed where the curved proximal part passes into the more straight distal 

 part. The chords of the clades are in the anaclades of var. megana 45-210 n, 

 in those of var. orthotriaena 33-80 [i, and in those of var. microana only 30-50 p. 

 long. The angles between the clade-chords and the axis of the rhabdome are 

 27-66°, on an average 47°. 



DIMENSIONS OF CLADES OF SIDONOPS ANGULATA.' 





Thickness of 

 rhabdome at the 

 cladome 



Lengtli of 

 clade-chords 



Angle between clade-chords and 

 axis of rhabdome 



o 







(A 



limits 



average 



Var. megana, lobose specimen 



11-39 



45-210 



35-65 



47 



Var. megana, massive specimen 



7-38 



48-142 



27-66 



45 



Var. microana 



10-18 



30-50 



50-62 



47 



Var. orthotriaena 



14-18 



33-80 



43-54 



48 



' Rhabdome length not known. 



