186 
HYALONEMA (HYALONEMA) AGASSIZI. 
deposited may be a simple point, a short rod, or a cross. The spheres with a 
cross-shaped centre (Plate 45, fig. 29) lead to those short-rayed tetractines 
(Plate 45, figs. 24, 25) which have been referred to above as transitions between 
the normal long-rayed, stout, basal spicules and the spheres; I am inclined to 
consider the spheres as derivates of these spicules. 
I have not seen any spheres in the preparations of form B and C, but I 
found some, similar to those of form A, in form D. 
The microhexactines and their derivates form a series commencing with 
regular equal-rayed hexactines and ending with diactines and monactines. They 
fall into two groups : — 1 , regular and irregular microhexactines proper, and 
2, diactine and monactine microhexactine-derivates. 
The microhexactines -proper (Plate 44, figs. 15, 16, 17b, 18-23, 25-30) have 
regularly disposed rays which enclose angles of 90° with their neighbours. The 
rays are conic and pointed. Their basal part is straight, their distal part nearly 
straight or curved more or less, sometimes considerably. In the forms E and F, 
where the microhexactines with the most strongly curved rays are found inter- 
mingled with the other, more straight-rayed forms, the degree of curvature 
appears to be in inverse proportion to the size of the spicule. The end-parts 
of opposite rays are usually curved in opposite directions. The rays of these 
spicules are beset with small backwardly directed spines. These are largest 
and most numerous on the middle-part of the ray; proximal ly they decrease 
in number, distally in size. It is also to be noted that these spines are on the 
whole much larger in the large (and straight-rayed) than in the small (and more 
curved-rayed) microhexactines. 
The microhexactines proper of form A (Plate 44, figs. 16, 17b, c, 18-20, 22, 
23, 30) are 50-144 /x in diameter and have rays 1.7-4 m thick at the base. The 
irregular forms are larger (longer) and have thicker rays than the regular. The 
difference in the length of the rays of the irregular forms is sometimes very con- 
siderable, the length of the shortest ray being occasionally only a ninth of that of 
the longest. 
In the other forms the microhexactines proper are similar and also in these 
the irregular ones are larger than the regular. The maximum diameter of the 
microhexactines proper measured was in form B (Plate 44, figs. 21, 25, 27) 
66-145 /x, in form C (Plate 44, fig. 26) 44-130 m, in form D (Plate 44, figs. 28, 29) 
48-114 fx, in form E 53-157 /x, and in form F 52-160 /x. 
The diactine and monactine microhexactine-derivates are by no means fre- 
quent. The diactine microhexactine-derivates of form A (Plate 44, fig. 24) are 
