HYALONEMA (PRIONEMA) PINULIFUSUM. 287 
supporting skeleton, smaller amphioxes with distinct central tyle, tylostyles, 
and, exceptionally, amphityles are met. : 
The ordinary amphiozes are fusiform, very blunt at the ends, 0.8-2.5 mm. 
long, 10-33 » 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 u 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 » thick near the middle. The central tyle is 12-30 u in transverse 
diameter, that is 5-16 » 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 4 at morphological centre, which lies somewhere 
between the middle of the length and the terminal tyle; transverse diameter 
of terminal tyle 13-24 4; thickness just below terminal tyle 10-16 uw, that is 
3-8 uw 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 u thick 
_ in the middle and just below one of the terminal tyles. Towards the other 
tyle it is attenuated to 15 4. The two terminal tyles are respectively 41 and 
22 » 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 uw in 
total diameter. The rays are straight or slightly curved, conical, fine-pointed, 
1.5-2.2 » 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 y» in diameter and have rays 2-3.7 u thick at the base. 
The amphidiscs, which are 15-470 uw 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 amphidise more than 
