HYALONEMA (HYALONEMA) TENUIFUSUM. 
227 
sharp-pointed. The whole spicule is entirely smooth. The largest of these 
spicules are quite similar to, and only slightly shorter than, the smallest centro- 
tyle amphioxes above described as megascleres, and they might indeed be con- 
sidered as small forms of these spicules. There is, however, a very conspicuous 
gap which lies between 2 n and 7 n in the biometrical frequency-curve of the 
thickness of all these spicules taken together. This gap makes the distinc- 
tion easy between those 2 ^ thick and thinner, above described as minute 
centrotyle amphioxes, and those 7 /x thick and thicker, above described as cen- 
trotyle amphiox megascleres. 
The rare minute tylostyles, which are perhaps, foreign, are 140-200 /i long. 
The tyle is not situated quite at the end, is oval in shape, 5 m in diameter, and 
roughened by minute spines. The ray is 2-3 n thick at the base, smooth, coni- 
cal, pointed, and straight, or slightly curved. 
The amphidiscs measured are 20-340 n long. Their biological length 
frequency-curve is interrupted by a large gap between 34 and 77 n, and by minor 
gaps the most conspicuous of which lies in the curve of specimen (a) between 
160 and 230 /x, and in the curve of ( b ) between 152 and 202 /x. The amphidiscs 
20-34 /x in length form, morphologically, a fairly homogeneous group; they are 
to be considered as micramphidiscs. The amphidiscs 77-340 ^ in length are 
morphologically not homogeneous, the small ones having slender and long 
anchors, particularly in specimen id), whilst the anchors of the large ones are 
stout and short. Since, however, the broad and narrow anchored forms are 
morphologically, connected by very numerous transitional forms intermediate 
in size, I think it best to consider all the amphidiscs 77-340 ^ long as one group 
of macramphidiscs. The larger (on the whole broad anchored) and the smaller 
(on the whole narrow anchored) macramphidiscs are distinguished biometri- 
cally by the gap above referred to in their length frequency-curve (for specimen 
(a) between 160 and 230 /x, and for specimen (6) between 152 and 202 id). In 
accordance with this gap I distinguish two kinds of macramphidiscs, small 
macramphidiscs (in specimen (a) 129-160 ^ long, in (6) 77-152 /d) and large 
macramphidiscs (in specimen (a) 230-330 ^ long, in (6) 202-340 /d ). 
The large macramphidiscs (Plate 67 , figs. 2-5, 15, 26; Plate 68 , fig. 1) are 
230-330 /d long in specimen (a). The shaft is straight and cylindrical, 5-11 n 
thick, apart from an abrupt thickening somewhere near the middle and a gradual 
thickening towards both ends. The central thickening (tyle) is 10-20 ^ in 
diameter, usually about twice as much as the shaft. It bears a verticil of fairly 
straight, truncate, cylindroconical spines which are vertical to the shaft or, 
