HYALONEMA (PRIONEMA) PINULTFUSUM. 
287 
supporting skeleton, smaller amphioxes with distinct central tyle, tylostyles, 
and, exceptionally, amphityles are met. 
The ordinary amphioxes are fusiform, very blunt at the ends, 0.8-2. 5 mm. 
long, 10-33 fi thick in their middle-part, and generally curved. In a good many 
of them, particularly the long ones, the curvature is very considerable. A few 
angularly bent spicules of this kind have also been observed. 
The small centrotyle amphioxes are generally 390-620 m long; but larger 
ones, connecting them with the amphioxes above described, have also been 
observed. The small centrotyle amphioxes are straight or only slightly curved, 
and 7-14 n thick near the middle. The central tyle is 12-30 /x in transverse 
diameter, that is 5-16 m more than the adjacent parts of the spicule. 
The tylostyles are amphiox-derivates with one ray reduced in length and 
terminally thickened. Their dimensions are: — total length 1.2-1. 7 mm.; 
maximum thickness 16-22 n at morphological centre, which lies somewhere 
between the middle of the length and the terminal tyle; transverse diameter 
of terminal tyle 13-24 /x; thickness just below terminal tyle 10-16 m, that is 
3-8 n less than the diameter of the terminal tyle. The terminal tyle is more 
or less spherical, and usually bears one or two small, stout, truncate or terminally 
rounded spines. Occasionally a rather large spine arises from it. 
The amphityles are amphiox-derivates with both rays reduced in length 
and terminally thickened. One that I measured is 1.7 mm. long, and 27 n thick 
in the middle and just below one of the terminal tyles. Towards the other 
tyle it is attenuated to 15 n. The two terminal tyles are respectively 41 and 
22 n in diameter. 
The microhexactines (Plate 71, figs. 1-4, 9) are regular, the six rays of the 
same spicule usually being fairly equal. These spicules measure 80-120 m in 
total diameter. The rays are straight or slightly curved, conical, fine-pointed, 
1.5-2. 2 \x thick at the base, and just perceptibly roughened by exceedingly 
minute spines. 
A few micropentactines (Plate 71, fig. 10) have been found, which appear 
to connect the regular microhexactines with the small canalar pinules. These 
spicules are 80-140 /x in diameter and have rays 2-3.7 m thick at the base. 
The ainphidiscs, which are 15-470 n long, exhibit a remarkable degree of 
diversity. I have measured 178 of them. Their length frequencies are repre- 
sented in Figure 14. 
The figure shows that the lengths of the amphidiscs form a nearly uninter- 
rupted series. At one point only we find the next largest amphidisc more than 
