356 HYALONEMA (OONEMA) SEQUOIA. 



since the extreme forms are connected by numerous transitions, I shall here 

 describe both together. 



The small macramphidiscs (Plate 86, figs. 7, 27-34; Plate 89, fig. 15d; 

 Plate 90, figs. 1-10) are 90-195 m long, most frequently about 164 fi. The 

 shaft is straight, smooth, cylindrical, and 5.5-16.5 yu thick. The terminal 

 anchors are 39-100 n long, and 55-174 /x broad, usually 70-150 m- The pro- 

 portion of anchor-length to anchor-breadth is 100 : 120 to 100 : 178, on an 

 average 100 : 149. As has been stated above, these anchors are about half as 

 long as the whole spicule, sometimes a little shorter than that, more frequently 

 a little longer. Each anchor is composed of five to thirteen teeth. The larger 

 amphidiscs of this kind, that is those to which the part of the length frequency- 

 curve culminating at 164 /x refers, have five to ten, usually eight teeth; the 

 smaller, to which the part of the curve culminating at 93 n refers, have eight to 

 thirteen, usually eleven teeth. The two terminal anchors of the same spicule 

 are composed of the same number of teeth. The teeth of the terminal anchors 

 extend in planes passing through the axis of the spicule. The anchor-teeth 

 planes of one anchor enclose equal angles with their neighbours, each 360 degrees 

 divided by the number of teeth. The anchor-teeth planes of the other anchor 

 of the same spicule alternate regularly with these in such manner that they 

 divide each angle into two equal parts (halves). Thus the tips of the teeth of 

 the two opposite anchors are not opposite but alternate. 



The individual anchor-teeth are curved, either uniformly or, more fre- 

 quently, less in the middle-part than at the base and at the tip. The outer 

 contour of each tooth is abruptly curved inwards at the distal end. The teeth 

 are T-shaped in transverse section. Their outer (upper) part, which corresponds 

 to the upper stroke of the T, has the shape of a curved band increasing in breadth 

 distally to a point three quarters of the length of the tooth from its base. Here 

 the tooth is 9-30 /j. broad. The end-part of the tooth, lying beyond this point 

 of maximum breadth, is simply rounded (Plate 90, figs. 1, 3, 7, 9). The inner 

 (lower) part of the tooth, which corresponds to the lower stroke of the T, is a 

 thick keel, uniformly decreasing in height distally. The end-part of the upper 

 (outer) band-shaped portion of the tooth bends down around the end-part of 

 the keel on all sides except the axial, so that, viewed in profile, the end-part of 

 the whole tooth becomes strikingly similar to a crow's beak (Plate 90, figs. 4, 6, 

 8, 10). 



Slightly abnormal small macramphidiscs with one or more somewhat irregu- 

 lar teeth, like the one represented (Plate 90, figs. 5, 6) , have repeatedly been met. 



