178 



GEODIA AMPHISTRONGYLA. 



actine forms of such, in which the thinner end has become attenuated to a fine 

 point. 



The plagiotriaenes (Plate 20, figs. 22, 25) have conical, usually quite straight 

 rhabdomes, 1.8-2.2 mm. long and 22-32 p. thick at the cladomal end. The 

 acladomal end is usually simply rounded off and 3-10 /x thick, rarely slightly 

 thickened. The clades are slightly curved, concave to the rhabdome, conical, 

 usually regularly arranged, with angles of 120° between them, and, in the same 

 cladome, of equal size and in nearly the same position to the rhabdome. The 

 clades are 155-190 /j. long and enclose angles of 103-120°, most frequently about 

 109°, with the rhabdome. Rarely the clade-rhabdome angles of the same 

 rhabdome are very dissimilar. In one plagiotriaene-cladome of this kind one 

 of the three clades was nearly upright, resembling in its position an epirhabd. 

 This sword-like spicule might be termed a mesorthodiaene. 



Among these plagiotriaenes similar forms with reduced cladomes, which 

 appear as plagiodiaenes or plagiomonaenes occur. The monaene forms are more 

 frequent than the diaene. The cladome of a normal monaene of this kind is 

 shown in Plate 20, fig. 23. Very rarely the rhabdome is reduced in length and 

 in that case cylindrical and slightly thickened at the acladomal end. One of 

 these spicules (Plate 20, fig. 24) has a rhabdome, only 285 /x long, and a single, 

 bifurcate clade, arising at an angle of hardly more than 90° from the rhabd- 

 ome. This spicule is an orthodichomonaene. Dichoclade forms of this or 

 any other description are, however, exceedingly rare. 



Owing to the loss of nearly the whole of the spicule-fur I found but few 

 mesoproclades (Plate 20, figs. 7, 8), which presumably form the greater part of 

 this fur. None of them had an intact rhabdome, so that I cannot give its length. 

 Most of the mesoproclades observed were triaene, but monaene forms (Plate 20, 

 fig. 7) have also been met with. The rhabdomes of these spicules are, at the 

 cladome, 3-5 /j. thick; the epirhabds are straight, conical, sharp pointed, and 

 16-22 p. long; the clades are concave to the epirhabd, usually rather blunt, and 

 40-60 // long. The clade-epirhabd angles of the intact cladomes observed 

 were 36-41°, but some broken mesoprotriaene cladomes seen indicate that 

 occasionally this angle is considerably larger. 



The anatriaenes (Plate 20, figs. 5, 6, 10, 11) are, like the mesoproclades, 

 scarce in the preparations, and probably for the same reason. Anatriaenes 

 with intact rhabdomes were not observed. Their rhabdomes are 1.5-4 p. thick 

 at the cladomal end, the clades are conical and sharp pointed, more (Plate 20, 

 figs. 5, 10) or less (Plate 20, figs. 6, 11) curved, concave to the rhabdome in their 



