88 



GEODIA ATAXASTRA. 



lation between the three cladome dimensions, they are subject to considerable, 

 apparently independent variations. 



Besides the orthoplagiotriaenes with regular cladomes, above described, 

 a few occur in which the clades are irregularly and unequally curved (Plate 43, 

 fig. 10). Very rarely monaene and amphiclade derivates of orthoplagiotriaenes 

 are met with. A monaene form observed had a bifurcate clade. An amphi- 

 clade possessed, besides the ordinary three terminal clades, a fourth clade, which 

 arose some distance down the rhabdome. 



The mesoprodades of var. angustana (Plate 43, fig. 24b; Plate 44, figs. 1- 

 12, 14) have a straight or shghtly curved rhabdome 1.6-3.4, mostly 2.1-2.8 mm. 

 long. At the cladome the rhabdome is 9-20 /x, most frequently 10-15 ft thick. 

 Farther down it thickens, and at its thickest point, which is situated near the 

 middle, it is usually half again as thick as at the cladomal end. At the acladomal 

 end the rhabdome is usually attenuated to a fine point. Generally the cladome 

 is regularly triaene. The clades are usually conical, about 70 p. long, slightly 

 curved, concave to the epirhabd basally and straight distally. Their chords 

 enclose angles of 40-50° with the epirhabd. The latter is straight, conical, 

 pointed, and 40-50 pt long. Besides these regular forms various irregular ones 

 are observed. In some of these a fourth clade is added to the three normal 

 ones, so that the spicule appears as a mesoprotetraene. The clades of these 

 tetraene mesoprodades (Plate 44, fig. 1) are somewhat unequal in length and 

 position. The longest clade of the cladome is 50-75 p. long. The chords of the 

 clades enclose angles of 36-62° with the axis of the epirhabd. The latter is 

 conical, pointed, and 35-58 p. long. Other derivates are produced by one or 

 more clades being abruptly (angularly) bent outward in the middle (Plate 44, 

 fig. 14). These spicules are also very rare. More frequently irregularities due 

 to one or two of the clades being reduced in length, cylindrical, and terminally 

 rounded, are observed (Plate 44, figs. 11, 14). Rarely one clade is absent 

 altogether, so that the spicule appears as a mesoprodiaene (Plate 44, fig. 12). 

 Taking all the mesoprodades together, but leaving out of account the excep- 

 tionally large, quite abnormal angles of some of the tetraene clades, which, 

 as mentioned above, measured up to 62°, we find that the longest clade of 

 the cladome is 44-80 p and the epirhabd 40-60 p long, the angle between the 

 clade-chords and the epirhabd axis being 33-48°, on an average 41.3°. 



In var. latana the mesoprodades (Plate 44, fig. 13) have a rhabdome 2-2.4 

 mm. long and, at the cladome, 7-13 p thick. As in the mesoprodades of var. 

 angustana the rhabdome thickens towards the middle. Regular mesoprotriaenes, 



