200 



GEODIA MEDIA. 



The stout pJagiotriaenes (Plate 16, figs. l-6s, lis, 12s) occur in large num- 

 bers in both forms. Their rhabdome is straight or very slightly curved, and 

 usually pointed, rarely rounded at. the acladomal end. It is either regularly 

 conical throughout or slightly constricted just below the cladome. The rhabd- 

 omes of five of the plagiotriaenes of the massive form thus constricted, which 

 I measured, were: — 



60 /! thick at the cladomal end and 62 /t thick at the thickest point a little farther down. 



60 " " " " " " •• 64 " " " " " " " " 



60 " " " " " " " 67 " " " " " " " " 



65 " " " " " " " 68 " " " " " " " " 



67 " " " " " " " 70 " " " " " " " " 



In observing a thin transverse splinter of a plagiotriaene-rhabdome with a 

 high power I found its axial rod to be triangular in transverse section. 



In the digitate form the rhabdomes of the stout plagiotriaenes are 0.8-1.4 

 mm. long and 30-65 p., usually 50-60 /t thick at the cladomal end. The clades 

 are only slightly curved, concave to the rhabdome, conical, not sharp pointed, 

 and 110-260 /t long. Their chords enclose angles of 107-118° with the axis of the 

 rhabdome. The stout plagiotriaenes of the massive form are similar in shape 

 but considerably larger. Their rhabdomes are 0.9-1.7 mm. long and at the cla- 

 domal end 30-80 /x, usually 50-70 /( thick. Their clades are 160-310 p. long and 

 the chords of their clades enclose angles of 105-120° with the axis of the rhabd- 

 ome. The length of the rhabdome and the clades is not in proportion to their 

 thickness, the very thick ones having by no means particularly long rhabdomes 

 or clades. 



In Bowerbank's type, examined by me, the dimensions of the stout plagio- 

 triaenes are: rhal:)domc 0.8-1.2 mm. by 40-60 /Jt, clades 200-260 fx long, clade- 

 angles 105-120°. Sollas gives their measurements as follows: rhabdome 1.12 

 mm. by 450 (printers error for 45) fi, clades 254 n long, clade-angles (according 

 to the relation of the clade-length to the transverse diameter of the cladome, 

 320 i-i, given by him) large. 



Besides the regular plagiotriaenes described above a few irregular derivates 

 of them occur. In some of these the clades are reduced in length and rounded 

 at the end; in others two of the clades are single and one bifurcate (Plate 16, 

 fig. 12t); in others again the clades are curved convex to the rhabdome (Plate 

 16, fig. 3u). I have also observed a few mesoclade plagiotriaene-derivates in 

 which the rhabdome is much shortened, rounded at the end, and continued be- 

 yond the cladome in the shape of a long conical epirhabd (Plate 16, fig. Iv). 



The mesomonaenes (Plate 16, fig. 7m, n, o, p; Plate 17, figs. 8-12), which 



