106 



GEODIA MESOTRIAENA. 



are caused by retardation of growth or irregular bending of one, two, or, rarely, 

 all three clades. The first leads to irregularities in the length of the clades, 

 the last to other irregularities of the cladome. Not only the clades but also 

 the epirhabd of the mesotriaenes may become rudimentary, whereby forms like 

 the one, represented in Plate 23, fig. 15, are reproduced. Such irregular telo- 

 clades are, however, rare. The clades are 90-310 /j. long; their chords form 

 angles of 29-54°, on an average 42.4° with the epirhabd-axis. The meso- 

 protriaenes of var. megana have longer clades than those of the other two 

 varieties. In the mesoprotriaenes of var. pachana the epirhabd is shorter than 

 in those of the two others. 



DIMENSIONS OF MESOPROTRIAENES OF GEODIA MESOTRIAENA. 





Var. pachana 



Var. megana 



Var. microana 





length mm. 



6-14 



8.8-11 





Rhabdotne 



thickness at 

 the cladome n 



15-40 



18-40 



17-22 





maximum thick- 

 ness /( 



70 



38-65 





Epirhabd 



length /t 



95-230 



140-330 



100-300 



Clades 



length /( 



90-230 



140-310 



130-210 



Angles between clade- 

 chords and epirhabd- 



maximum and 

 minimum ° 



29-56 



35-53 



35-54 



axis 



average ° 



43.7 



43.5 



40 • 



The anatriaenes (Plate 23, figs. 1-12) have a rhabdome 11-16 mm. long, 

 22-40 f.i thick at the cladomal end, and attenuated to a fine, more or less 

 twisted thread at the acladomal end. The clade-chords are 90-270 pi long and 

 enclose angles of 34- 58° with the axis of the rhabdome. They are curved, con- 

 cave towards the rhabdome, either (Plate 23, figs. 3, 6) quite uniformly or (Plate 

 23, figs. 10-12) more strongly between the first and second third of their length 

 than elsewhere. There is always a small protuberance on the summit of the 

 cladome. The clades of the anatriaenes of var. megana are considerably longer 

 than those of the other two varieties. The rhabdomes and clades of var. mi- 

 croana are considerably thinner than those of the other two varieties. 



Rarely irregular, mesoclade anatriaene-derivates (Plates 23, fig. 17) with 

 three regular anatriaene-clades and a curved epirhabd are met with. In their 



