190 GEODIA ACANTHTYLASTRA. 



In the centrifugal spicule-p reparations I found some very minute ampJiioxes 

 41-53 /( long and 1-1.2 ft thick. These may belong to the sponge, but it is more 

 probable that they are foreign. 



The plagioiriaenes (Plate 46, figs. 18b, 19b, 30-35, 38) have a fairly straight 

 rhalxlome. Its cladomal half is usually nearly cylindrical, its acladomal half 

 conic and pointed (Plate 46, figs. 18b, 19b, 31-33). Rarely the rhabdome is 

 nearly cylindrical throughout and rounded at the acladomal end (Plate 45, fig. 

 38). The rhabdome is 1.2-2.5 mm., in fully developed plagiotriaenes usually 

 about 2.1 mm. long and, at the cladome, 40-77 fi thick. The cladome is usually 

 simple and composed of three fairly equal stout, conic, and blunt -pointed clades, 

 curved, concave to the rhabdome quite strongly in their proximal part. The dis- 

 tal part of the clades is slightly curved in the same direction, or straight (Plate 

 ^6, figs. 18b, 19b, 30-32, 34, 35, 38). The clades of these regular plagiotriaenes 

 are 160-260 fi long, their chords enclosing angles of 100-116°, on an average 

 106°, with the axis of the rhabdome. Besides these spicules, which form the 

 great majority, similar ones with irregular cladomes have been observed in 

 small numbers. In these spicules either a fourth clade, situated some distance 

 below the cladome, is added to the three ordinary terminal ones, or one (Plate 

 46, fig. 33) or more clades are bifurcate or provided with a short, irregular, 

 upward-directed branch. In the spicule-preparations also some true dicho- 

 triaenes (Plate 46, figs. 36, 37), with shorter rhabdomes and smaller clade-angles 

 than those of the regular plagiotriaenes were seen. As, however, I failed to find 

 regular dichotriacnes in the sections the}' may very likelj' not belong to the sponge. 



The mesoprotriaenes (Plate 46, figs. 1-7; Plate 46, fig. 20d) have a straight 

 or slightly curved rhabdome, 2.3-3.3 mm. l(jng. At the cladome it is 13-22 p., 

 farther tlown, at its thickest point near the middle, 21-31 p, usually not quite 

 half again a.s thick as at the cladomal end. This thickening towards the middle is 

 exemplified by the following measurements of five mesoprotriane-rhabdomes: — 



THICKNESS OF THE RH.\BDOME. 



