RHABDOCALYPTÜS VICTOR. 249 



average 200 /^ in length. Sometimes two gastralia may lie close 

 together, in which case the directly adjoining gastral laths are 

 three rays strong instead of two as usual. Unusually small and 

 slender-rayed oxyhexactins occasionally present in the layer are 

 without doubt gastralia which have not yet attained complete 

 development. 



Taking part in the formation of gastral laths, diactins are 

 not infrequently found, the rays of which are generally somewhat 

 thinner but Ioniser than those of the liexactins. The center is 

 marked either by an annular swelling or by four cruciately dis- 

 posed knobs ; the surface may be nearly smooth but is more 

 generally roughened by the presence of microtubercles in varying 

 numbers. There can scarcely be any impropriety in classing some 

 at least of the diactins under the gastralia ; at the same time 

 they may be looked upon as spicules linking the gastralia proper 

 to the hypogastralia. 



Oxyhexasters (PI. XYIIL, figs. 5, 7 and 15) of a large 

 size, measuring 180-280 /^ in diameter, are abundant in the 

 choanosome. Normal forms are rather scarce ; more frequently 

 are the oxyhexasters hexactinose and most commonly, hemihex- 

 actinose. The principals are extremely short or nearly obsolete, 

 making the terminals almost appear to radiate directly from the 

 common central node. The terminals are strong, measuring up to 4 

 /^ in thickness at base ; their surface is always rough, the roughness 

 developing into small barbs in the basal parts. The uniterminal 

 principal is either straight or bent at its junction with the 

 terminal. In the former case the atrophied terminal may some- 

 times be represented by a unilateral boss (fig. 7). 



In the normal oxvhexasters the number of terminals to each 



