HYALONEMA (PRIONEMA) PINTLIFUSUM. 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 /i 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 // 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 m 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 yu 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 /i, that is 

 3-8 M 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 raj's reduced in length 

 and terminally thickened. One that I measured is 1.7 mm. long, and 27 fi thick 

 in the middle and just below one of the terminal tyles. Towards the other 

 tyle it is attenuated to 15 /i. The two terminal tyles are respectively 41 and 

 22 fi in diameter. 



The microhexadines (Plate 71, figs. 1-4, 9) are regular, the six rays of the 

 same spicule usually being fairly equal. These spicules measure 80-120 ^ in 

 total diameter. The rays are straight or slightly curved, conical^ fine-pointed, 

 1.5-2.2 IX 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 ^ in diameter and have rays 2-3.7 ^ thick at the base. 



The amphidiscs, which are 15-470 yu long, exhilsit 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 



