226 HYALONEMA (HYALONEMA) TENUIFUSUM. 



In specimen (b) I found a microhexactine with a branch-ray on one of its 

 rays. 



In the irregular microhexadines (Plate 67, fig. 8; Plate 68, figs. 7-9, f2-15) 

 two opposite rays, lying in the same spicular axis, are long and well-developed, 

 the other four variously reduced. The two long rays may be considered as 

 apical, the other four as lateral rays. The two long apical rays are considerably 

 longer than the rays of the regular microhexactines, so that the maximum diam- 

 eter, that is the total length of these spicules, exceeds the diameter of the regular 

 microhexactines. The apical rays are straight or only A'ery slightly curved in 

 their basal and middle-part, conical, pointed, smooth, or slightly roughened by 

 very minute spines, and are 1-2.2 n thick at the base. The degree of reduction 

 of the four lateral rays is equal or unequal, and is generally very considerable. 

 They may all be present and equally long, or one, two, or three of them may 

 be shorter or altogether absent. When one or more of these rays have dis- 

 appeared altogether, the remaining lateral rays are usually very short. Spic- 

 ules in which three of the lateral rays have disappeared altogether whilst the 

 fourth is only shghtly reduced in length (Plate 68, fig. 13) are very rare. It is 

 to be noted that the reduced lateral rays are not only shorter, but often also 

 thinner than the apical, the difference in the basal thickness of the apicals and 

 laterals often amounting to 0.5 fi. 



The irregular microhexactines in both specimens are 125-400 ju long and 

 8-112 II broad. There is a very clearly pronounced correlation between the 

 length of the apical rays (the total length of the spicule) and the degree of 

 reduction of the lateral rays (the total breadth of the spicule) ; the longer the 

 apicals and the whole spicule, the shorter are the laterals and the narrower is the 

 whole spicule. 



The irregular microhexactines 125-170 long are 72-112 broad. 



190-220 " " 33-85 

 280-400 " " 8-14 



Thus these irregular microhexactines form a series connecting the (shorter) 

 regular microhexactines described above with the (longer) minute centrotyle 

 amphioxes described below. 



The minute centrotyle amphioxes (Plate 67, fig. 12; Plate 68, figs. 16, 17) 

 are more or less cur\'ed, the central part usually in one direction, the two end- 

 parts in the opposite, so that these spicules generally look like bows. They are 

 mostly 580-830 n long, and 1.5-2 ju thick near the middle. The central tyle is 

 o\al and measures 3-5 m in trans\'erse diameter. The two rays are conical and 



