GEODIA MESOTRIAENELLA. 



153 



Some intact rhabdomes of the mesoprotriaenes (Plate 35, figs. 28-30, 31b) 

 were 2.8-3.4 mm. long, but several of the fragments observed appeared to be parts 

 of rhabdomes longer than that. The thickness of the rhabdome at the cladome 

 is 9-19 iL The rhabdomes which are thick at the cladome are in their cladomal 

 half nearly cylindrical, hardly perceptibly thickened towards the middle; the 

 rhabdomes thin at the cladome, on the other hand, are very considerably thick- 

 ened towards the middle of their length. The epirhabd is straight, conical, 

 pointed, and 70-165, usually 90-120 [x long. The clades are slender, conical, 

 and curved, concave to the epirhabd. Their chords are 100-220 p. long and 

 enclose angles of 30-47°, on an average 36°, with the axis of the epirhabd. The 

 clades of the same cladome are equal or, more frequently, unec^ual in length 

 (Plate 35, figs. 28-30); all of them or, if they differ considerably in length, two, 

 or at least one, are longer than the epirhabd. The proportion of the length 

 of the epirhabd to the length of the longest clade of the same cladome being 

 100 : 130 to 100 : 233. 



The anatriaenes (Plate 35, figs. 32-35). The longest intact rhabdome 

 measured was 3.7 mm. long. Fragments indicate that many anatriaenes have 

 rhabdomes considerably longer. The thickness of the rhabdome at the cladome 

 is 18-30, usually 20-22 /«. The clades are conical, pointed, uniformly and not 

 strongly curved, concave to the rhabdome. Their chords are 87-140, usually 

 110-130 n long and enclose angles of 41-57°, on an average 48°, with the axis of 

 the rhabdome. Besides these normal anatriaenes with simple clades I have 

 observed a few, similar to them in all other respects, with one of the clades 

 bifurcate. 



The large oxyasters (Plate 34, figs. 18a, 21, 22, 24) have a small centrum the 

 diameter of which is usually about double the basal thickness of the rays. From 

 this centrum from five to eleven fairly regularly distributed rays arise radially. 

 The rays are straight, conical, usually quite blunt, 9-14 /t long, and, at the base, 

 1.5-2.8 /I thick. The basal parts of the rays are smooth, the distal parts covered 

 with spines (Plate 34, figs. 21, 22). The size of these spines is variable and their 

 number is, roughly speaking, in inverse proportion to their size. The largest 

 spines measured were 0.7 ft long. The spines arise vertically from the ray. 

 Those large enough to be properly seen are usually sharp pointed, but occasion- 

 ally I have also observed stout and cylindrical, terminally rounded spines. 

 The whole oxy aster is generally 17-26 in diameter. In the centrifugal 

 spicule-preparations also large ones, up to 40 fi in diameter, have been ob- 

 served. As, however, I never found such large asters in situ in the sections 



