78 I. IJIMA : HEXACTINELLIDA. I. 



possible that further research with more materials may reveal 

 the presence of the rosette in that species also. 



The sieve-plate presents a spiculation somewhat differentiated 

 from that of the lateral wall. The principal parenchynialia are 

 here oxydiactins, smooth at the center and gradually attenuating 

 toward either end. They may attain 30 mm. or over in length 

 and 190/^ in breadth at the center. They are usually more or 

 less curved and often rather abruptly bent in accommodation to 

 the corners of the sieve-plate meshes. The accessoria, copiously 

 present and forming close bundles with the pi-incipalia, are likewise 

 predominantly diactins and occasionally thetactins. The diactin 

 forms show either annulated or cruciately tubercled centers. 

 Certain diactins are remarkably short in relation to their thick- 

 ness and may be called compass-needle-like. The external surface 

 of the sieve-plate exhibits nearly regular hexactin-dcrnialia of 

 150-400/^ axial length, sparingly present on the thinner beams 

 but densely crowded on the nodal plates. The rays in these 

 spicules are relatively strong and acutely or bluntly pointed at 

 the roughened ends ; the proximal ray pierces right into the sub- 

 jacent parenchymal bundle. The gastralia on the inner surface 

 are less abundant. They are pentactins of an appearance quite 

 similar to the dermalia save the absence of the free ray. All 

 the three forms of rosettes found in the lateral wall occur in the 

 sieve-plate also, — the fioricome and the oxyhexaster very sparingly 

 but the graphiocome in abundance. Hence, free rhaphides are 

 of quite common occurrence in the dermal layer. Finally, it 

 may here be added that I have ascertained by sections the 

 presence of the chamber-layer in the sieve-plate. 



