SIDONOPS ANGULATA. 



29 



12, fig. 16b). In the angularly bent spicules the angular bend amounts to 7- 

 12°, and is usually, as in the spicule represented in Plate 12, fig. 16b, not in the 

 middle, but considerably nearer one end than the other, the two straight parts 

 of the spicule thus enclosing an angle of 168-173° and being unequal in length. 

 The slender dermal amphioxes are fairly isoactine, thickest in the middle, 

 gradually attenuated towards the ends, and terminally abruptly and sharply 

 pointed. They are 2.9-9.5 nun. long and 5-34 n thick. Those of var. ortho- 

 triaena are much smaller than those of the others. 



DIMENSIONS OF SLENDER DERMAL AMPHIOXES OF SIDONOPS ANGULATA. 



Styles and style-derivates (Plate 13, fig. 20a) were not found in the massive 

 specimen of var. megana at all and in the three other specimens they are very 

 rare. The regular styles are straight, conic, sharp pointed at one end, and 

 considerably thickened and club shaped at the other. They are 2.1-2.5 mm. long 

 and 60-100 n thick at the rounded end. In var. microana I found a style-derivate 

 2.3 mm. long and 110 // thick at the rounded end, which bore, 400 fi below the 

 rounded end, a terminally rounded branch-ray, 120 n long, directed obliquely 

 towards the rounded end of the main shaft. 



The plagio- and ortho-triaenes (Plate 12, figs. 16d, 17d, 21, 22; Plate 13, 

 figs. 3-16, 18, 19) have a straight or shghtly curved rhabdome, which is usually 

 on the whole conical, and attenuated towards the pointed acladomal end, more 

 rapidly in the acladomal terminal part than in the cladomal and central parts. 

 These normal plagio- and ortho-triaene-rhabdomes are 1.5-2.8 mm. long and 

 47-82 fJL thick at the cladomal end. In var. orthotriaena I found two abnormal 

 orthotriaenes, one with a cyhndrical, terminally rounded, and considerably 

 shortened and thickened rhabdome only 550 ft long but 105 fi thick, and another 

 with an angularly bent rhabdome (Plate 13, fig. 19). The angle enclosed 

 between the cladomal and acladomal parts of the latter is 109°. The clades 

 are conic and usually pointed, rarely (Plate 13, fig. 8) rounded at the end. 

 They are 330-700 p. long, those of the plagiotriaenes of var. megana being a 



