GEODIA MESOTRIAENA. 



105 



is in the other two on an average 99.4° and 104.2° respectively. Thus these 

 spicules are mostly orthotriaenes in var. megana, but partly (var. pachana 

 or mostly (var. microana) plagiotriaenes in the other two. By the complete 

 suppression of one or two of the clades ortho- and plagio-diaenes and monaenes 

 are produced. These are, however, rare. A plagiomonaene which I found 

 among the spicules of var. megana had a rhabdome 100 fj. thick at the cladomal 

 end, and a clade 530 fx long, the chord of which enclosed, with the rhabdome- 

 axis, an angle of 102°. 



DIMENSIONS OF ORTHO- AND PLAGIO-TRIAENES OF GEODIA MESOTRIAENA. 



The meso'protriaenes (Plate 21, fig. 6; Plate 23, figs. 13, 14, 18, 19) have 

 a rhabdome 6-14 mm. long. It is thickest in the middle. Here it measures 

 38-70 n in transverse diameter and from here it tapers towards both the cladomal 

 and the acladomal end. At the cladome it is usually half as thick or less than 

 at its thickest point near the middle of its length, and here measures only 15- 

 40 ^ in transverse diameter. As an example of this I give the following measure- 

 ments of a mesoprotriacne of var. megana. The rhabdome of this spicule was 

 10.5 mm. long. At the thickest point, which was situated 4.8 mm. below the 

 cladome, it was 59 n, and at the cladome 27 fi thick. The epirhabd is 95-330 /x 

 long and usually simple, straight, conic, and pointed (Plate 23, figs. 14, 18, 19). 

 Sometimes (Plate 23, fig. 13) it bears branches, forming an imperfect secondary 

 cladome above the cladome proper. The clades are not nearly so constant in 

 shape as the epirhabd, and mesoprotriaenes with irregular clades of very frequent 

 occurrence. In the most regular mesoprotriaenes (Plate 23, figs. 18, 19) the 

 clades are conical, pointed, or blunt, and more or less curved, concave towards 

 the epirhabd. Although this curvature appears rather uniform in the clades 

 themselves, the observation of their axial rods shows clearly that it is in reality 

 greatest at the base and decreases distally. The irregularities of the cladomes 



