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; proximally 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 ^ 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 M, in form C (Plate 44, fig. 26) 44-130 m, in form D (Plate 44, figs. 28, 29) 

 48-114 n, in form E 53-157 n, and in form F 52-160 n. 



The diactine and monactine microhexactine-derivates are by no means fre- 

 quent. The diactine microhexactine-derivates of form A (Plate 44, fig. 24) are 



