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 ^ and 7 ^ i" the biometrical frequency-curve of the 

 thickness of all these spicules taken together. This gap makes the distinc- 

 tion easy between those 2 ix thick and thinner, above described as minute 

 centrotyle ampliioxes, and those 7 ^ thick and thicker, above described as cen- 

 trotj^e amphiox megascleres. 



The rare minute tylostyles, which are perhaps, foreign, are 140-200 ix long. 

 The tyle is not situated quite at the end, is oval in shape, 5 yu in diameter, and 

 roughened by minute spines. The ray is 2-3 m thick at the base, smooth, coni- 

 cal, pointed, and straight, or slightly curved. 



The amphidiscs measured are 20-340 /i long. Their biological length 

 frequency-curve is interrupted by a large gap between 34 and 77 m, and by minor 

 gaps the most conspicuous of which lies in the curve of specimen (a) between 

 160 and 230 ai, and in the curve of (6) between 152 and 202 //. The amphidiscs 

 20-34 M 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 (b), 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 yu 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-cur\-e (for specimen 

 (a) between 160 and 230 n, and for specimen (b) between 152 and 202 ix). In 

 accordance with this gap I distinguish two kinds of macramphidiscs, small 

 macramphidiscs (in specimen (a) 129-160 m long, in (b) 77-152 /x) and large 

 macramphidiscs (in specimen (a) 230-330 ^ long, in (b) 202-340 m)- 



The large macramphidiscs (Plate 67, figs. 2-5, 15, 26; Plate 68, fig. 1) are 

 230-330 fi long in specimen (a). The shaft is straight and cylindrical, 5-11 ^ 

 thick, apart from an abrupt thickening somewhere near the middle and a gradual 

 thickening towards both ends. The central thickening (tyle) is 10-20 /i in 

 diameter, usually about twice as much as the shaft. It bears a verticil of fairly 

 straight, truncate, cylindroeonical spines which are vertical to the shaft or, 



