166 HYALONEMA (HYALONEMA) OBTUSUM. 



two opposite longer, and four shorter rays, with which the first series commences, 

 is represented in Plate 40, fig. 9. This spicule is 117 ^ in length. Forms still 

 farther removed from the regular microhexactine are produced by a further 

 reduction of the four shorter rays of such a spicule. The reduction of the four 

 shorter rays is either unequal or more or less equal. In the first case pentactines, 

 tetractines, and triactines (Plate 40, fig. 11) with two opposite longer rays, and 

 three, two or only one shorter, are produced ; in the second case forms like those 

 represented on Plate 35, figs. 20-22, and Plate 40, figs. 12, 13, and 20c. In the 

 extreme forms of this series all that remains of the shorter rays is a slight tyle 

 (Plate 35, fig. 22; Plate 40, fig. 12). It is to be noted that a distinct increase 

 in size of the two opposite, developed rays is, in these spicules, associated with 

 the reduction of the four other rays. Such diactine microhexactine-derivates 

 are, particularly in var. gracilis, more numerous than .any of the others. They 

 are in both varieties 156-204 m long, but in var. robusta considerably stouter than 

 in var. gracilis, the basal parts of their properly developed rays being in the former 

 1.5^ M, while in the latter only 1.5-2.5 m thick. The fully developed rays of 

 these spicules are gradually attenuated to fine points, straight in their basal part 

 and curved at the end. The reduced ones are straight throughout, cylindrical 

 or cyhndroconic, terminally rounded, and reach 6 ^i in length. The terminal 

 curvature of the fully developed rays is not so great as in the rays of the regular 

 microhexactines, nor is its direction generally opposite. 



To the second series of microhexactine-derivates belong the spicules repre- 

 sented on Plate 40, figs. 8, 10, 14, and 15. The first (fig. 8) of these is a pentactine 

 with equal rays, 100 m in diameter. The second (fig. 10) is a compass-shaped 

 diactine. It consists of two fully developed rays, 47 m long, the basal parts of 

 which enclose a right angle; and the insignificant rudiments of two other rays 

 opposite to the two fully developed ones. The third and fourth (figs. 14, 15) 

 are monactines. Such monactines are more frequent than the other forms of 

 this series. They are 73-86 m long. Their single fully developed ray is 2.5-4 m 

 thick at the base and tapers gradually to a fine point. It is straight in its basal 

 part but strongly curved, through an angle of about 120°, in its distal part. 



These spicules are, like the regular microhexactines, covered with minute 

 spines. In the larger ones the spines are more conspicuous than in the smaller 

 ones, the size of the spines being, generally speaking, proportional to the thick- 

 ness of the ray from which they arise. 



The pachy7nicrohexactines (Plate 39, figs. 7-10) are rather rare, and have 

 only been found in the basal part of var. gracilis. I consider them as hypertrophic 



