ERYLUS CALICULATUS. 



313 



number. This proportional correlation is, however, not a uniformly regular one, 

 for while the three- to five-rayed acanthtylasters and their parts are nearly equal 

 in size, and the same applies to the seven- and eight-rayed ones, there are con- 

 siderable dimensional differences between the two- and three-rayed, the five- 

 and six-rayed, and six- and seven-rayed ones. In a curve representing this 

 correlation two steep, step-like falls would interrupt the general descent. The 

 diniensions of the two-rayed, three- to five-rayed, six-rayed, and seven- to eight- 

 rayed acanthtylasters are tabulated below. 



.\CANTHTYLASTERS. 



Many oxyasters, similar in diameter and ray-number to the acanthtylasters, 

 occur in the choanosome. The rays of these spicules are quite slender in their 

 distal jmrt but usually thickened in a very marked manner at their base. Simi- 

 lar spicules with slightly tliicker, rough rays, and others with still thicker and 

 rougher rays and a slight terminal thickening, connect the oxyasters with the 

 acanthtylasters. I consitlcr the former as young stages of the latter. 



The small oxysphaerasters (Plate 6, fig. 3; Plate 7, figs. 11-15, 74c) have 

 a spherical centrum, 4-5.5 pi in diameter, from which from ten to twenty or more 

 equal, concentric, regularly distributed, conical, blunt or sharp-pointed rays arise. 

 The rays are, at the base, 0.7-1.4 pt thick, and smooth. Their middle and distal 

 parts are covered with small spines. Occasionally a few spines, larger than the 

 rest, form a loose verticil some distance from the end of the ray. The whole 

 aster is 9-18 p in diameter. A correlation (inverse proportion) between size and 

 ray-number is not discernible. 



The aspidasters (Plate 6, figs. 19-22; Plate 7, figs. 31-41; Plate 8, figs. 1, 

 15-20) are discs varying from broad-oval to circular or reniform. Very rarely 

 aspidasters quite irregular in outline, with several deep incisions, have been 

 observed. The broad-oval to circular aspidasters (Plate 6, figs. 19, 20; Plate 7, 

 figs. 31-37), which are much more numerous than the reniform ones, are fairly 

 regular in outhne, 72-88 fi long, 67-77 n broad, and 10.6-12.8 n thick. The 



