292 HYALONEMA (PRIONEMA) PINLTLIFUSUM. 



The large smooth mesamphidiscs (Plate 72, figs. 3-6) are 122-292 fx long. 

 Their length frequency-curve has three nearly equal elevations at about 150, 

 180, and 240 ix. The shaft is nearly always straight, only in two of the hundreds 

 observed was it markedly curved. The shaft is for the greater part of its length 

 cylindrical, and 4-9 ix thick. It is abruptly thickened at or near the middle 

 to a central tyle 7-18 ^ in transverse diameter, that is 2-11 n more than the 

 adjacent parts of the shaft. The tyle bears a number of spines which are gen- 

 erally arranged in a more or less verticillate manner, sometimes however scat- 

 tered indiscriminately over the whole tyle. When these spines form a verticil, 

 this is situated in a plane either vertical or oblique to the axis of the shaft. 

 The indi\'idual spines are usually strongly curved or abruptly bent in their 

 distal part, cylindroconical, and truncate or terminally rounded. They are 

 2-1 1 IX long, and about 2 y, thick at the base. The remaining parts of the shaft 

 generally bear numerous small, and often also several large spines, 4-6 ^ long, 

 exceptionally as much as 10 ^u. The small spines usually arise vertically; most 

 of the large ones are oblique, directed and also curved in a distal direction 

 (towards the adjacent anchors, away from the central tyle). 



The anchors are, as in the large serrated amphidiscs, either regular or irreg- 

 ular, oblique, with the teeth on one side longer than the teeth on the other. 

 The two anchors of the same spicule are generally fairly equal in size ; sometimes, 

 however, one is considerably larger than the other. In one of the large smooth 

 mesamphidiscs observed, one anchor measured 133 by 78 y., the other only 

 112 by 62 /i. The anchors are 47-133 ix long, usually over two fifths of the 

 whole spicule, and 26-78 m broad. The proportion of their length to their 

 breadth is 100 to 49-73, on an average 100 : 57.9. The anchors are composed 

 of ten to twelve teeth. The individual teeth are curved, more strongly in their 

 basal part and often also in their distal than in their middle-part. Their total 

 curvature is such that their ends are either parallel or only slightly convergent 

 or divergent. In the forms with terminally convergent teeth the end-breadth 

 of the anchor is sometimes 7 ix less than its maximum breadth. 



The small smooth mesamphidiscs (Plate 72, figs. 7, 8) are similar to the 

 large ones, but have not so sharply defined central tyles, and relatively broader 

 anchors. Their dimensions are: — length 84-120 yu, most frequently about 

 110 /Li; thickness of shaft 2-5 y,; diameter of central tyle 3.4-9 /x, that is 0.6-4 m 

 more than that of the adjacent parts of the shaft; anchor-length 30-46 /x; anchor- 

 breadth 21-40 ix\ proportion of anchor-length to anchor-breadth 100 to 60-87, 

 on an average 100 : 73. The anchors of these spicules are composed of about 

 ten teeth. 



